High Plasma Progesterone Levels Research Articles

Abstract 007: Endothelial Mineralocorticoid Receptors are Increased by Pregnancy in Mice and Mediate Obesity-Associated, Leptin-Induced Endothelial Dysfunction in Pregnancy

Obesity is a risk factor for endothelial dysfunction and hypertension in pregnancy. Our lab has demonstrated that obesity-associated hypertension in female animal models develops via leptin-mediated, aldosterone-induced endothelial dysfunction (ED). We also showed that female mice are more sensitive to leptin-induced aldosterone activation of endothelial mineralocorticoid receptors (ECMR) due to an endogenous sex-specific upregulation of ECMR in females. Our previous data demonstrates that endothelial progesterone receptor activation increases ECMR expression and that ECMR deletion protects female mice from obesity-associated, leptin-mediated ED. However, whether ECMR play a role in ED in pregnancy is unknown. Therefore, we hypothesized that high progesterone levels increase ECMR expression in pregnant mice and that ECMR deletion protects pregnant mice from obesity-associated, leptin-mediated ED. Endothelial cells were isolated from aorta of timed-pregnant Balb/C mice (gestation day (GD) 17) and assessed for ECMR expression via RT-PCR. ECMR mRNA expression increased 9.2±0.2-fold (*P<0.05) in pregnant compared to nonpregnant mice in association with high plasma progesterone levels (24±6 ng/ml pregnant vs 2±2 nonpregnant, *P<0.05). Endothelial function was experimentally measured by wire myography concentration response curves to acetylcholine in mouse aorta analyzed by 2-way ANOVA of curves with repeated measures. No changes in endothelial-independent sodium nitroprusside responses were observed. Chronic leptin infusion in mid-pregnancy (GD11-17) induced ED in pregnant mice that was impaired both compared to sham-treated pregnant (*P<0.05) and non-pregnant leptin-infused females (*P<0.05), indicating that increased ECMR expression in pregnancy increases sensitivity to leptin-induced ED. Further evidence for this notion is provided in that ECMR deletion in pregnant mice protected them from leptin-mediated ED (*P<0.05). Collectively, these data indicate a role for ECMR in the pathogenesis of obesity-associated ED in pregnancy and indicate a potential role for MR antagonists for the treatment of hypertensive pregnancies such as preeclampsia.

The progesterone derivative dydrogesterone down-regulates neurokinin 1 receptor expression on lymphocytes, induces a Th2 skew and exerts hypoalgesic effects in mice

Accumulating evidence indicates that the neuropeptide substance P (SP) is predominantly involved in neurogenic inflammation and pain perception via its high-affinity neurokinin 1 receptor (NK-1R). Intriguingly, decreased pain sensitivity is found to be associated with high plasma progesterone levels. We hypothesize that progesterone may attenuate nociception and associated inflammatory response via NK-1R-dependent pathways. To address our hypothesis, we incubated splenic lymphocytes from CBA/J female mice with different concentrations of the progesterone derivative dydrogesterone. Subsequently, the expressions of NK-1R and T helper (Th1)-type cytokines were analyzed by flow cytometry. Next, we subcutaneously injected CBA/J mice with 1.25 mg of dydrogesterone in 200-microl sesame oil; control mice were sham-injected. Tail flick test to detect the nociceptive threshold was performed in 30-min intervals upon injection. Lymphocytes were isolated from blood and uterus and analyzed for NK-1R surface expression. Immunohistochemical analyses were performed to investigate the uterine tissue distribution of NK-1R. Dydrogesterone induced a decrease in the percentage of NK-1R+ lymphocytes in vitro and in vivo. Additionally, an increase in Th2-type and a decrease in Th1-type cytokines could be detected in vitro after incubation with dydrogesterone. An increased tail flick latency following dydrogesterone injection supported the concept that decreased expression of the NK-1R on lymphocytes is associated with an increased pain threshold. Taken together, these results clearly reveal a pathway by which dydrogesterone or progesterone respectively modulates the cross talk of the nervous, endocrine and immune systems in inflammation and pain.

Increased extracellular local levels of estradiol in normal breast in vivo during the luteal phase of the menstrual cycle

Estrogen exposure is a major risk factor for breast cancer. Tissue estrogen originates from the ovaries but a significant portion is also produced by enzyme activity locally in the breast itself. How these enzymes are regulated is not fully understood. The extracellular space, where the metabolic exchange and cell interactions take place, reflects the environment that surrounds the epithelium but there has been no previous study of hormone concentrations in this compartment. In the present study microdialysis was used to measure extracellular estrogen concentrations in breast tissue and abdominal subcutaneous fat in 12 healthy women in vivo. It was found that women with high plasma progesterone levels had significant increased levels of estradiol in breast tissue compared with fat tissue (breast tissue 168+/-6 pM; subcutaneous fat, 154+/-5 pM; P<0.05), whereas women with low plasma progesterone exhibited no difference. Moreover, there was a significant correlation between local breast tissue estradiol and plasma progesterone levels (r=0.709, P<0.01). There was no difference in estrone sulphate in breast and fat tissue regardless of progesterone levels. Estrone was not detectable. The results in this study suggest that progesterone may be one regulator in the local conversion of estrogen precursors into potent estradiol in normal breast tissue.

Reduced LH sensitivity in vivo and in vitro of corpora lutea induced during anoestrus by GnRH, and during the late breeding season, in Scottish Blackface ewes

Scottish Blackface ewes were synchronised in mid-breeding (November; group 1; n=12 ewes) or late-breeding season (March; group 2; n=16). Anoestrous ewes (May) were treated with progestagen sponges for 7 days and then given 250 ng GnRH 3-hourly for 24 h, 2-hourly for 24 h and hourly for a further 24 h (group 3; n=12). A second group of anoestrous ewes (group 4, n=19) received three bolus injections (30 microg) of GnRH at 90-min intervals without progestagen pretreatment. After ovulation, ewes were bled twice daily until slaughter (day 4 or day 12: oestrus=day 0). Mid-breeding season (group 1) and anoestrous ewes in group 3 formed 'adequate' corpora lutea (CL) with high plasma progesterone levels (3-4 ng/ml) maintained for at least 12 days, and responded in vivo to ovine LH (oLH) (10 microg) with a rise in plasma progesterone on day 11 (group 3, but not group 1, ewes also responded on day 3). CL minces from these ewes responded to human chorionic gonadotrophin (hCG) in vitro with a dose-dependent increase in progesterone secretion. Ewes in group 4 had a foreshortened luteal phase (8-10 days) and low plasma progesterone levels (approximately 1 ng/ml), consistent with formation of inadequate CL. LH injection failed to induce a significant plasma progesterone increase. Furthermore, although progesterone secretion in vitro in response to maximally stimulating doses of hCG or dibutyryl cAMP (dbcAMP) was similar to that in adequate CL, the sensitivity of these CL to hCG (EC (effective concentration)50, 1 IU hCG/ml) was reduced 10-fold compared with adequate CL (EC50, 0.1 IU hCG/ml; P<0.01). Ewes that ovulated in the late breeding season (group 2) had high plasma progesterone, although levels began to decrease after day 10. Injection of oLH in vivo increased plasma progesterone. However, sensitivity to hCG in vitro (EC50, 0.5 IU hCG/ml) was intermediate between that of adequate luteal tissue (groups 1 and 3; EC50, 0.1 IU/ml) and that of group 4 ewes (EC50, 1 IU hCG/ml). Our data demonstrate a markedly reduced luteal sensitivity to LH in vivo and hCG in vitro in Scottish Blackface ewes with inadequate CL, and suggest that a similar loss of sensitivity to LH may occur in the late breeding season.

High local endometrial effect of vaginal progesterone gel

Our objective was to investigate the first-pass effect to the uterus of progesterone gel administered vaginally. This was a prospective, randomized study of 32 postmenopausal women, attending our menopause clinic. All women used transdermal estradiol (50 μg/day, a patch each week) for 2 weeks. They used either vaginal progesterone gel or intramuscular progesterone in oil 50 mg after 24 h to oppose the transdermal estradiol. Serum progesterone levels and endometrial tissue progesterone levels were determined. Serum progesterone levels were higher in women who used the intramuscular rather than the vaginal route. Although serum progesterone levels in the vaginal group were lower than in the intramuscular group, the endometrial tissue concentration of progesterone was higher. It is concluded that progesterone gel, used vaginally, has a high local effect on the endometrium, without any systemic side-effects due to high plasma progesterone levels.

Predicting the onset of parturition in the goat by determining progesterone levels by enzyme immunoassay

There are currently a few methods that can be used to assess the time of impending parturition in the goat, especially if exact breeding dates are unknown. One indicator of the onset of parturition is a decrease in the doe’s blood progesterone level approximately 24–30 h prior to parturition. Laboratory serum/plasma progesterone assays commonly require 24 h or more to obtain results, as well as being expensive. This study was designed to evaluate whether semi-quantitative enzyme immunoassay (EIA) progesterone kits could be used to determine plasma progesterone levels in does during late gestation, and whether a correlation could be drawn between the test result and the time of parturition. Plasma samples were collected twice daily from 16 pregnant does starting on day 143 of pregnancy and ending 1 day following parturition. The plasma progesterone levels indicated by the three EIA kits were compared to one another and to a radioimmunoassay, and assessed for any relationship to the actual time of parturition. Test results indicating low plasma progesterone levels (<1.0–2.8 ng/ml) from the Status Pro (Synbiotics), TARGET (Biometallics), and PreMate (Camelot Farms) and tests were found to be correlated 100, 83 and 100% with the onset of parturition within 24 h. Test results indicating high plasma progesterone levels (>5.0–7.8 ng/ml) from the Status Pro, TARGET, and PreMate tests were found to be correlated 100, 100, and 82.2% with delayed parturition. The radioimmunoassay results concurred with the progesterone ranges given for Status Pro, TARGET, and PreMate tests in 75.7, 58.8 and 53.7% of the cases, respectively. The results of this study indicates that any one of the commercially produced EIA progesterone tests can be used to predict with a high degree of accuracy the likelihood of parturition occurring within the next 24–30 h. These tests are simple to perform and cost effective.

Apoptosis in decidual tissue regression and reorganization.

During blastocyst implantation, cellular degeneration of decidual tissue spreads from the antimesometrial region to the mesometrial region to accommodate the developing conceptus. To identify the mechanism of decidual regression and its tissue localization, decidual formation was induced in either intact pseudopregnant rats or ovariectomized rats treated with steroids. Antimesometrial and mesometrial cells were separated by elutriation and cultured for 48 h before DNA analysis. Nucleosomal DNA fragmentation was observed in antimesometrial cells, whereas DNA fragmentation was less marked in mesometrial cells. To determine whether a similar pattern of apoptosis occurred during decidual development in vivo, decidua tissues from antimesometrial and mesometrial regions were obtained on days 8-14. Nucleosomal DNA fragmentation was first detected on day 10 in the antimesometrial region, with fragmentation in the mesometrial region delayed by at least 24 h. DNA fragmentation increased in both tissues with time, but was always more pronounced in antimesometrial cells. Sulfated glycoprotein-2 (SGP-2) has been associated with apoptosis, and its expression was examined by Northern analysis. Levels of SGP-2 messenger RNA (mRNA) were detected in the antimesometrial region on day 9 and increased markedly by day 10; mesometrial expression was delayed 24 h. Levels of SGP-2 mRNA in both regions decreased before the onset of DNA fragmentation. In contrast, cathepsin-D expression, detected by immunohistochemistry, was localized to few mesometrial and antimesometrial cells between days 9-12, with all cells showing extensive staining by day 14. To determine whether changes in progesterone control mechanisms initiated decidual apoptosis, plasma progesterone levels were measured by RIA, and progesterone receptor mRNA levels in decidual tissues were examined by reverse transcription-polymerase chain reaction. Both parameters remained unchanged during decidual growth and regression. These results provide biochemical evidence that decidual regression occurs by apoptosis initiated in the antimesometrial region and with progression to the mesometrial region. Apoptotic cell death occurs despite high levels of plasma progesterone and high levels of progesterone receptor message in decidual tissue.

Apoptosis in decidual tissue regression and reorganization

During blastocyst implantation, cellular degeneration of decidual tissue spreads from the antimesometrial region to the mesometrial region to accommodate the developing conceptus. To identify the mechanism of decidual regression and its tissue localization, decidual formation was induced in either intact pseudopregnant rats or ovariectomized rats treated with steroids. Antimesometrial and mesometrial cells were separated by elutriation and cultured for 48 h before DNA analysis. Nucleosomal DNA fragmentation was observed in antimesometrial cells, whereas DNA fragmentation was less marked in mesometrial cells. To determine whether a similar pattern of apoptosis occurred during decidual development in vivo, decidua tissues from antimesometrial and mesometrial regions were obtained on days 8-14. Nucleosomal DNA fragmentation was first detected on day 10 in the antimesometrial region, with fragmentation in the mesometrial region delayed by at least 24 h. DNA fragmentation increased in both tissues with time, but was always more pronounced in antimesometrial cells. Sulfated glycoprotein-2 (SGP-2) has been associated with apoptosis, and its expression was examined by Northern analysis. Levels of SGP-2 messenger RNA (mRNA) were detected in the antimesometrial region on day 9 and increased markedly by day 10; mesometrial expression was delayed 24 h. Levels of SGP-2 mRNA in both regions decreased before the onset of DNA fragmentation. In contrast, cathepsin-D expression, detected by immunohistochemistry, was localized to few mesometrial and antimesometrial cells between days 9-12, with all cells showing extensive staining by day 14. To determine whether changes in progesterone control mechanisms initiated decidual apoptosis, plasma progesterone levels were measured by RIA, and progesterone receptor mRNA levels in decidual tissues were examined by reverse transcription-polymerase chain reaction. Both parameters remained unchanged during decidual growth and regression. These results provide biochemical evidence that decidual regression occurs by apoptosis initiated in the antimesometrial region and with progression to the mesometrial region. Apoptotic cell death occurs despite high levels of plasma progesterone and high levels of progesterone receptor message in decidual tissue.

Changes in Body Weight, Egg Production, Hackle Growth, and Plasma Sex Steroid Hormones and Prolactin during the Annual Reproductive Cycle in Domestic Geese.

This study was conducted to elucidate the changes in body weight, feather molts, hackle (cervical tract feather) growth, and plasma sex steroid hormones and prolactin in relation to annual reproductive cycle in adult domestic geese. Maximum body weight was observed in January (three of nine geese started laying), and gradually decreased during laying period (January to June). A positive correlation was observed between changes in food intake and body weight (rs=0.81, P<0.01). Plasma concentration of estradiol started to increase in January and reached maximum levels in coincidence with the highest egg production which occurred in May. Estradiol concentrations showed a positive correlation with egg production (rs=0.84, P<0.01). High levels of plasma concentrations of testosterone were observed from November to April. High levels of plasma progesterone were relatively confined to the second half of the laying period (April to June). Maximum prolaction concentration in plasma was observed in May, and was probably associated with broody behavior in two of the nine geese; this value was significantly greater than those observed in February, March, September and October (P<0.01). Changes in hackle growth showed a negative correlation with egg production (rs=-0.90, P<0.01). The nadir in hackle growth was clearly coincident with maximal egg production.

Relationships between puberty and production traits in the gilt. 2. Oestrous symptoms at puberty

The aim of this study was to investigate the relationships between oestrous symptoms at puberty and production traits. The gilts studied were of Swedish Yorkshire breed and belonged to a selection experiment for lean tissue growth rate. They were penned indoors in groups of six. Oestrus was checked twice daily from 160 to 260 days of age in the presence of a boar. Oestrous symptoms were carefully recorded. Blood samples for progesterone were taken regularly and puberty was defined as the first ovulation, as determined by plasma progesterone levels. Of the 547 gilts included in the study, 481 reached puberty during the experimental period. Of these, 77 gilts did not show the standing reflex at their first ovulation. However, most of the gilts had a red and swollen vulva. Gilts with low backfat thickness (high lean percentage) had less intense and shorter reddening and swelling of the vulva at puberty than gilts with high backfat thickness (low lean percentage). The backfat thickness had no influence on the ability to show the standing reflex or on the length of standing oestrus at puberty. The growth rate did not influence oestrous symptoms at puberty. The backfat thickness also influenced the levels of plasma progesterone after the first ovulation. Gilts with high backfat thickness had high levels of plasma progesterone. Age at puberty was found to have an influence on plasma progesterone levels and on vulvar symptoms. Gilts with early puberty showed intense and long durations of reddening and swelling of the vulva, and high plasma progesterone levels.

Transfer of porcine embryos after 3 days of in vitro culture.

Two experiments were conducted to determine the viability of porcine embryos transferred after long-term in vitro culture. In Exp. 1, four-cell embryos were kept in culture for 120 h. Embryos that were exposed to fresh culture medium every 12 h survived better than embryos kept in the same medium throughout the culture period. In Exp. 2, four- and eight-cell embryos were cultured in vitro for 72 h before transfer to estrus-induced recipient gilts. Each gilt received, on average, 19 embryos. If recipients were synchronous with donors 3/32 (9%) recipients remained pregnant with an average of 4.0 +/- .6 viable young. If the sexual cycle of the recipients was 24 h behind that of the donors the pregnancy rate was 18/34 (53%) with 4.4 +/- .5 viable young. Average embryo survival rate for the two groups was 1.8 and 12.5%, respectively. A 24-hourly medium replacement during the in vitro culture period had no significant effect on transfer results. When transferring freshly collected blastocysts, pregnancy rate, number of viable young and survival rate of embryos were 6/10 (60%), 7.8 +/- 1.4, and 23.9% for synchronous recipients and 7/10 (70%), 9.3 +/- 1.8, and 32.9% for asynchronous recipients, respectively. Recipients with very high plasma progesterone levels or numerous follicular cysts at the time of transfer were less likely to remain pregnant than others.

Pituitary and ovarian hormones in cows with ovarian cysts.

Gonadotropin profiles as well as sex steroids in the plasma of cows with ovarian cysts or cystic corpus luteum were examined. Bioactive inhibin levels and concentrations of sex steroids in the individual cystic fluid were determined to analyze the type of ovarian cyst. Plasma concentrations of LH in cows with follicular cysts were generally high as compared with basal LH levels in normal cows during the estrous cycle, though high levels of plasma estradiol-17β were noted. LH levels in cows with follicular cysts and corpus luteum, with luteal cyst or with cystic corpus luteum, however, were low similar to the basal levels of LH in the normal estrous cycle, being associated with high levels of plasma progesterone. Plasma concentrations of FSH in cows with any type of ovarian cyst, on the other hand, were within the range of basal levels of the normal estrous cycle, with no correlation to plasma levels of steroids.These results indicate that elevated levels of LH and basal levels of FSH within the normal range in the estrous cycle are characteristic in cows with follicular cysts. The possibility of involvement of inhibin in lowering FSH to basal levels in these cows with ovarian cysts is discussed.

Changes in plasma gonadotropins, ovarian steroids and inhibin concentrations in gilts following progesterone treatment with implantable osmotic pumps

Endocrine changes in cyclic gilts (10 months old, n=5) were examined during and following the measured administration of progesterone with implantable osmotic pumps, commencing during the late luteal phase of a cycle. Exogenous progesterone delivered from the pumps (115 mg/day) maintained high plasma progesterone levels (> 10 ng/ml) that effectively suppressed follicular growth and prevented the gilts from returning into estrus. Thus, 7 days progesterone implantation resulted in an extension of the estrous cycle by about a week. Progesterone withdrawal initiated the increase of plasma inhibin as well as of estradiol and the decrease of plasma FSH. In four out of five treated gilts, the exception being one gilt that developed follicular cysts, the LH surge was observed 6.3±0.3 (s.e.m.) days after the removal of progesterone implants. The overall plasma inhibin profiles were inversely correlated with those of FSH: during follicular development, inhibin levels were high while plasma FSH remained low, then, after the LH surge, a decline of plasma inhibin preceded the secondary surge of FSH around the time of ovulation. The method herein described appears to be a useful tool for synchronizing the preovulatory endocrine events in gilts.

Effect of pulsatile infusion of gonadotropin-releasing hormone on plasma oestradiol-17 beta concentrations and follicular development during naturally and artificially maintained high levels of plasma progesterone in heifers.

To stimulate a follicular-phase pattern of pulsatile LH release, gonadotrophin-releasing hormone (GnRH; 5 micrograms) was infused (i.v.) hourly into heifers for periods of 5-11 days during the luteal phase of the oestrous cycle, and also when plasma progesterone levels were increased artificially by means of a progesterone-releasing intravaginal device. Plasma oestradiol-17 beta concentrations increased from basal (less than or equal to 2.5 pmol/l) to preovulatory peak levels (20-30 pmol/l) during the first 3 days of GnRH treatment. They were maintained at these values before returning to basal levels within 24 h of cessation of infusion. This response occurred regardless of the source of progesterone (endogenous or administered). Follicular development was observed by ovarian palpation (per rectum) in some heifers at the time of maximum secretion of oestradiol-17 beta. There was no detectable cervical mucus secretion or oestrous behaviour during these periods of high oestradiol-17 beta levels and ovulation did not occur. Treatment with GnRH did not affect plasma progesterone concentrations or oestrous cycle length. The study shows that oestradiol-17 beta secretion and follicular development (and the accompanying oestrus and ovulation) are suppressed during the luteal phase of the cycle by high concentrations of plasma progesterone, and provides strong indirect evidence that such inhibition is associated with a reduction in the pulse frequency of LH release.

Prolongation of diestrus after removal of pups in lactating rats.

The recurrence of diestrus following removal of pups on day 10 of lactation was studied in lactating rats. Pituitary responsiveness to LHRH, and plasma LH, prolactin and progesterone levels were determined after removal of pups. Diestrous periods following romoval of pups were 8.61±0.36 and 3.33±0.21 days in the females which had been nursing 12 and 2 pups, respectively. Seventy-eight % of animals that had nursed 12 pups exhibited typical vaginal smear of pro-estrus on day 2 or 3 of post-weaning, but none of these animals ovulated on the following day. The number of follicles over 550μm in diameter rapidly in-creased from day 1 (1.7±1.2) to day 2 of post-weaning (7.6±1.9) in females that had nursed 12 pups. On day 2 or 3 of post-weaning obvious LH surge occurred in females that had nursed 2 pups, but not in animals that had nursed 12 pups. The rise of plasma LH levels following LHRH injection was less and the ovulatory responsiveness to LHRH injection was lower in females nursing 12 pups than those in rats nursing 2 pups on day 2 or 3 of post-weaning. High plasma progesterone levels were maintained until day 6 of post-weaning in animals that had nursed 12 pups, while concentration decreased to the basal level within 18 h after removal of pups in females nursing 2 pups. In accordance with the high level of plasma progesterone, the fluctuation of prolactin levels was observed following removal of pups in animals that had nursed 12 pups. It was suggested that the prolongation of the diestrous period following removal of pups in the rats nursing a large litter was attributable to the reduced responsiveness of pituitary to LHRH and to the maintenance of functional corpora lutea formed at post-partum ovulation.

Multiple progesterone injections and the duration of estrus in ovariectomized guinea pigs

The duration of sexual receptivity in guinea pigs primed with 3.3 ug estradiol benzoate and given 0.1 mg progesterone 36 hr later in oil or propylene glycol vehicle was about 6–7 hr. Maintenance of elevated plasma progesterone levels (as measured by radioimmunoassay)throughout the period of heat by multiple 0.1 mg progesterone injections significantly prolonged (by about 2.7 hr) heat duration, but heat terminated despite the presence of high plasma progesterone levels. The experiment suggests that a decline in plasma progesterone concentration is not necessary for the termination of heat. Methods by which termination of heat could occur are discussed.

Concentrations of oestradiol-17 beta, oestrone and progesterone in jugular venous plasma of cows during the oestrous cycle and in early pregnancy.

ABSTRACT Plasma oestradiol-17β concentrations rise slowly in the late progestational phase of the bovine oestrous cycle before plasma progesterone levels fall and then rise rapidly to a pre-oestrous peak of about 6 pg/ml as progesterone levels fall rapidly. The oestradiol-17β level (on average) falls during oestrus and both steroids show minimum levels 1–2 days after oestrus. Both then rise again, oestradiol-17β to a second peak 6–7 days after the pre-oestrous peak as progesterone level continues to rise. In a cow in the first 30 days of pregnancy, when plasma progesterone levels remained generally high after the 15th day, the pattern of plasma oestradiol-17β levels was very similar to that expected had it not conceived. Thus in the cycle and the first 30 days of pregnancy the main oestradiol-17β peaks are found at about 21 day intervals, each being followed by a smaller peak 6–7 days later. Either oestradiol-17β peak may assume a pre-oestrous and pre-ovulatory role in the absence of high plasma progesterone levels.

Interaction between prolactin, LH, neurohypophysial hormones and hysterectomy on luteal function in rats.

Functional corpora lutea (CL), which were capable of sustaining high levels of plasma progesterone, were maintained in hypophysectomized rats by transplantation of the pituitary or by daily treatment with exogenous prolactin. Treatment of autografted rats with exogenous LH (20 μg/day) for 6 days greatly reduced the plasma concentration of progesterone. Injections of LH to autografted rats for only 2 days markedly reduced progesterone in one half of the experimental animals. Even though exogenous LH was clearly luteolytic in autografted rats, the response in prolactin-treated, hypophysectomized rats depended upon the exact timing of each LH injection relative to each prolactin injection. Exogenous LH for 6 days greatly decreased plasma progesterone when each LH injection (1100 and 2000 hr) occurred 3 hr after a prolactin injection (0800 and 1700 hr). Injection of LH 3 hr before each prolactin injection had no effect on the concentration of plasma progesterone which was the same as in controls treated only with prolactin. Simultaneous injection (0800 and 1700 hr) of LH and prolactin produced an intermediate effect characterized by a slight reduction in progesterone concentration in some rats. The permissive effects of endogenous prolactin and of exogenous prolactin injected 3 hr before LH were manifested in the ability of exogenous LH, given for several days, to induce functional luteolysis and decrease plasma progesterone. Treatment of autografted rats with large dosages of neurohypophyseal hormones did not affect the plasma concentration of progesterone. Hysterectomy of pituitary intact, pseudopregnant rats had no effect on plasma progesterone on days 8 and 10. Plasma progesterone in hysterectomized animals remained elevated on days 12 and 14 whereas the levels in sham-operated controls declined.