Preovulatory Luteinizing Hormone Surge Research Articles

Evidence for direct estrogen regulation of the human gonadotropin-releasing hormone gene.

This study is an attempt to determine whether estrogen could directly regulate human gonadotropin-releasing hormone (GnRH) gene expression. Human GnRH expression vectors were constructed by fusing various 5' flanking regions of the human GnRH gene upstream of the luciferase reporter gene (LUC) or the thymidine kinase promoter linked to the chloramphenicol acetyltransferase reporter gene (CAT). These constructs were transiently transfected into a human choriocarcinoma cell line (JEG-3) and LUC or CAT activity was measured after either no treatment or treatment with various concentrations of estradiol. A stimulatory estrogen response element (ERE) was localized to a 32-bp region between -547 and -516 bp. To determine whether estrogen receptor bound to this region of the gene, we performed DNase I footprinting using purified calf uterine estrogen receptor. DNase I footprinting demonstrates a strong footprint between -567 and -514 bp of the human GnRH gene. In addition, an avidin-biotin complex DNA-binding assay demonstrated that a biotinylated DNA fragment containing -541 to -517 bp of the human GnRH gene bound 35S-labeled estrogen receptor as well as a biotinylated DNA fragment containing the xenopus vitellogenin ERE. On the other hand, the negative control biotinylated DNA fragment derived from adenovirus 5 bound insignificant amounts of 35S-labeled estrogen receptor. Both the GnRH ERE and vitellogenin ERE bound 35S-labeled estrogen receptor with high affinity (approximately 1 nM). These data indicate that the human GnRH gene contains an ERE sufficient to mediate a stimulatory response to estrogen in heterologous cells. Based upon these data we hypothesize that the human GnRH gene might also be directly regulated by estrogen in the hypothalamus, and that this regulation may explain the GnRH hypersecretion observed at the time of the preovulatory luteinizing hormone (LH) surge.

The mode of GABA B receptor-mediated inhibition of the preovulatory luteinizing hormone surge in female rats

In estrogen-primed ovariectomized rats, intraventricular injections of baclofen, a selective GABA B receptor agonist, either delayed, eliminated or disrupted the steroid-induced LH surge, depending on the time and the dose of the agonist injected. The delay in the onset of the LH surge was not due to a phase delay in the circadian clock, since the retarded LH surge, when observed, had a short duration, terminating in the evening simultaneously with the termination of the normal LH surge in control animals. Rather, rapid disruption of the ongoing LH surge after baclofen injection during the surge suggested a direct involvement of the GABA B receptor-mediated inhibitory component in the LH surge mechanism.

Hormonal control of proceptive and receptive sexual behavior and the preovulatory LH surge in the ewe: Reassessment of the respective roles of estradiol, testosterone, and progesterone

When estrous behavior is induced in ovariectomized ewes by subjecting them to progestagen priming followed by a dose of estrogen large enough to guarantee estrus in all animals, an abnormally long period of estrus in induced, suggesting that the regime of steroid replacement needs modification. Using quantitative tests for proceptivity and receptivity, we studied the patterns of sexual behavior of intact ewes and then attempted to reproduce them in the same animals after they had been ovariectomized. We used various combinations of exogenous estrogen, androgen, and progestagen and compared the behavioral responses with an endocrine response, the preovulatory surge of luteinizing hormone (LH). In intact ewes, sexual behavior and the LH surge were closely synchronized and their characteristics differed slightly between the middle and the end of the breeding season. Proceptive behavior was not greatly affected by the frequency of tests, but the duration of receptivity was significantly reduced by frequent testing. In ovariectomized ewes, we found that: (a) progesterone priming is essential for normal patterns of receptive and proceptive behavior, and for synchronizing the behavioral and endocrine responses to estrogen; (b) androgens do not play a major role in the control of either receptive or proceptive behavior; and (c) the inclusion of a low dose of estrogen with the progestagen in the ‘priming’ regime improves the responses to estradiol-17β. Under these conditions, the timing, intensity and duration of the behavior are very close to those observed in the same ewes when they were intact and cycling spontaneously.

Changes in the Release of Gamma‐Aminobutyric Acid and Catecholamines in the Preoptic/Septal Area Prior to and During the Preovulatory Surge of Luteinizing Hormone in the Ewe

Abstract The technique of intracranial microdialysis was used to monitor changes in the outflow of the catecholamines, noradrenaline, adrenaline and dopamine and the inhibitory amino-acid gamma-aminobutyric acid (GABA) in the preoptic/septal area of the conscious ewe during an oestradiol-induced surge of luteinizing hormone (LH). The same animals were sampled twice from an identical brain site, once in the presence of oestradiol and once in its absence, when no surge occurred and LH levels remained low. Changes in the outflow of GABA, noradrenaline and adrenaline (but not dopamine) were related to changes in LH secretion. Specifically, GABA outflow was maximal in the hours following oestradiol administration but began a sustained fall some 10 h before the surge began, to level off just before the first increment in LH secretion. Low GABA concentrations were maintained until after gonadotrophin levels had once more returned to baseline. The release of all three catecholamines was pulsatile. Noradrenergic activity was greater in the presence of oestradiol although activity did not alter over the 20 h of sampling. The pulse frequency of adrenaline was maximal in the hours immediately prior to the LH surge and minimal in the hours following its initiation. These data suggest that a decrease in GABAergic transmission in the vicinity of the LH-releasing hormone cell bodies is a necessary component of the neural mechanism by which the oestradiol-induced surge of LH is generated. A general increase in noradrenergic activity coupled with changes in the release of adrenaline at the time of the surge may be additional prerequisites for successful ovulation.

Induction of preovulatory luteinizing hormone surge and prevention of ovarian hyperstimulation syndrome by gonadotropin-releasing hormone agonist

To use gonadotropin-releasing hormone agonist (GnRH-a) instead of human chorionic gonadotropin (hCG) to induce oocyte maturation for in vitro fertilization (IVF). Pituitary and ovarian responses to GnRH-a and the outcome of IVF were studied prospectively. Data from patients injected with hCG were analyzed retrospectively. Program of IVF at the Rambam (Governmental) Hospital, Haifa, Israel. One or two doses of buserelin acetate 250 to 500 micrograms were administered to six patients with moderate response (Estradiol [E2], 1,494 +/- 422 [+/- SD] pg/mL) and 8 patients with exaggerated response (E2, 7,673 +/- 3,028 pg/mL) to gonadotropin stimulation. Progesterone (P) and E2 were administered for luteal support. Gonadotropin-releasing hormone agonist effectively triggered luteinizing hormone (LH)/follicle-stimulating hormone (FSH) surge. Mature oocytes were recovered in all patients. Luteal E2 and P were lower than in patients injected with hCG. No signs of ovarian hyperstimulation syndrome were observed. Serum LH and FSH rose over 4 and 12 hours, respectively, and were significantly (P less than 0.05) elevated for 24 hours. Of all mature oocytes, 67% fertilized and 82% cleaved. Four pregnancies were obtained. A bolus of GnRH-a is able to trigger an adequate midcycle LH/FSH surge, resulting in oocyte maturation and pregnancy. Our preliminary results also suggest that it allows a more accurate control of ovarian steroid levels during the luteal phase and may prevent the clinical manifestation of ovarian hyperstimulation syndrome.

Effects of reducing the remating interval after parturition on the fertility and plasma concentrations of luteinizing hormone, prolactin, oestradiol-17 and progesterone in lactating domestic rabbits

Primiparous crossbred does were remated on Day 1 (n = 15) or 14 (n = 25) post partum and killed on Day 10 post coitum to assess their fertility. Blood samples were taken during the pre- (0-12 h post coitum) and post- (1-10 days post coitum) ovulatory periods and plasma was assayed for luteinizing hormone (LH), prolactin, oestradiol-17 beta and progesterone. Ovulation response was significantly greater (P less than 0.01) and ovulation rate significantly lower (P less than 0.001) in does mated on Day 1 than in those mated on Day 14 post partum. Does failing to ovulate on Day 14 post partum exhibited no preovulatory LH surge and had significantly lower (P less than 0.05) premating concentrations of oestradiol-17 beta and prolactin than those ovulating at this time. No significant differences in hormone concentrations were observed during the preovulatory period between does ovulating on Days 1 and 14 post partum, with the exception of oestradiol-17 beta. Concentrations of this hormone were significantly lower (P less than 0.01) in does mated on Day 1, at 1 h post coitum. We conclude that (i) fertility was affected by the remating interval after parturition, (ii) ovulation failure was associated with an absence of the preovulatory LH surge and a reduction in premating concentrations of oestradiol-17 beta and prolactin and (iii) the lower ovulation rate in early lactation was apparently caused by a reduction in ovarian competence to respond to the gonadotrophic stimulus.

Transient Expression of Progesterone Receptor Messenger RNA in Ovarian Granuiosa Cells after the Preovulatory Luteinizing Hormone Surge

The ovarian steroid progesterone affects reproductive physiology by regulating the expression of specific genes in target tissues. In an attempt to address the question of whether the ovary itself is a target tissue for progesterone action, we have examined the localization and regulation of progesterone receptor (PR) mRNA in the rat ovary. We used the polymerase chain reaction to clone the steroid-binding domain of the rat PR from uterine cDNA and used this as a probe to isolate a larger cDNA from a rat placental cDNA library. We used RNA filter hybridization, a quantitative reverse transcription-polymerase chain reaction amplification assay, and in situ hybridization to detect PR mRNA in the rat ovary. Expression of the PR gene was initially studied in an immature animal model; 23-day-old rats were treated with either PMSG or PMSG followed by hCG. We found little or no PR mRNA in the ovaries of control or PMSG-treated animals; however, the mRNA was highly expressed in the granulosa cells of large follicles in the ovaries of animals treated with PMSG followed by hCG. Other cell types, including thecal and interstitial cells, did not express detectable levels of PR mRNA. The PR mRNA was induced more than 20-fold in the immature ovary 5 h after hCG administration and was down-regulated to near-basal levels by 12 h after hCG administration. In a subsequent series of experiments, we examined PR gene expression in adult rats during the estrous cycle. The expression of PR mRNA was transient and was tightly coupled to the preovulatory LH surge on proestrous evening. PR mRNA was localized to the granulosa cells of mature ovarian follicles during the estrous cycle. In cycling animals treated with pentobarbital to block the preovulatory LH surge, no induction of PR mRNA on proestrous evening was observed. This transient, hormonally regulated, and cell-specific expression of the PR gene in the rat ovary strongly suggests an important intraovarian function for progesterone during the rat reproductive cycle.

Bovine corpus luteum function after removal of granulosa cells from the preovulatory follicle

Experiments were conducted to determine the effects of removing granulosa cells from bovine preovulatory follicles on subsequent corpus luteum (CL) function. Holstein heifers were assigned to three groups: untreated controls (n=6), removal of granulosa cells (n=9) and removal and return of granulosa cells (n=7). Surgery was performed 18–24 hr after the onset of estrus and in all cases after the preovulatory luteinizing hormone (LH) surge. Jugular venous blood was collected and estrous activity monitored twice daily. Corpora lutea were formed in six heifers in each group. Concentrations of plasma progesterone were reduced (P<0.05) on Days 7–17 in heifers from which granulosa cells were removed when compared to the other two groups. There were no differences in the lengths of the estrous cycles nor concentrations of LH in the three groups. In summary, these experiments support the concept that granulosa cells make a substantial contribution to the output of progesterone by the cyclic CL but may have a limited role in determining the functional lifespan of the CL. These experiments also establish the fact that granulosa cells develop into functional luteal cells after their removal and return to the preovulatory follicle.

Estrus induction in the bitch using a combination diethylstilbestrol and FSH-P

The purpose of the study was to induce estrus and ovulation in normal bitches using a combination of diethylstilbestrol (DES) and follicle stimulating hormone of porcine pituitary origin (FSH-P). Thirteen mature mongrel female dogs were divided into two groups, the first group was treated for estrus induction during late anestrus and the second group during mid-anestrus. The dogs were monitored by teasing, vaginal cytology, and hormonal assay during the induced (n = 13) and the previous spontaneous estrous cycle (n = 9). Six of eight and three of five bitches came into standing estrus in the first and second group, respectively. Of the bitches that came into estrus, three conceived in the first group and one in the second. The average induced litter size was 7.0 versus 7.5 for the colony. Based on vaginal cytology the induced proestrus and estrus lasted 1.7 (0 to 3) and 12.9 (4 to 24) d, respectively, while the spontaneous proestrus and estrus lasted 5.8 (0–17) and 12.8 (9–15) d, respectively. Progesterone profiles were similar between the induced and spontaneous estrous cycles, although the progesterone peak was higher during the spontaneous cycle. The preovulatory luteinizing hormone (LH) surge was observed in only one induced estrous cycle. Modest results were obtained with this therapy. However, the litter sizes were normal and the induced cycles were very similar to the physiologic ones. No side effects were seen with the oral form of DES.

Effects of human chorionic gonadotropin (hCG) pretreatment on endocrine and behavioural responses of dairy cattle to exogenous prostaglandin F 2α

Sixteen heifers were used in two experiments to examine the effects of exogenous luteotropic stimulation on endocrine and behavioural responses to prostaglandin F 2α (PGF 2α). In each experiment, eight heifers were randomly assigned at synchronized oestrus to either receive 2500 IU of human chorionic gonadotropin (hCG) 48 h later or be maintained as controls. Blood samples for plasma progesterone (P) determinations were taken every 12 h from oestrus until PGF 2α administration on Day 7 (Experiment 1) or Day 13 (Experiment 2). After PGF 2α, samples were collected every 6 h for the next 36 h, and then every 2 h from 38 to 72 h. These samples were analyzed for plasma (P), luteinizing hormone (LH), and oestradiol 17 β (E 2) by radioimmunoassay. Administration of hCG 48 h postoestrus did not affect plasma P levels prior to PGF 2α administration in either experiment. In contrast, plasma E 2 levels were higher in control than hCG-treated heifers from 12 to 50 h post-PGF 2α in Experiment 1 and 12–68 h in Experiment 2. All eight control heifers exhibited a preovulatory LH surge during the first 72 h following PGF 2α administration, while one of the four hCG-treated heifers in each experiment had an increase in plasma LH levels. Of the hCG-treated group, one of four heifers in Experiment 1 and two of four heifers in Experiment 2 were observed in oestrus during the 96 h following PGF 2α, while all control heifers exhibited oestrual behaviour during this period. Exogenous administration of hCG during the early luteal phase did not affect subsequent luteolytic responses of the corpus luteum to PGF 2α, but did significantly reduce plasma E 2 levels, LH secretion, and delayed behavioural oestrus responses following PGF 2α.

Exposure to estradiol impairs luteinizing hormone function during aging

The loss of ovulatory cyclicity in many mammals is caused by changes in the hypothalamic-pituitary (H-P) control of the preovulatory luteinizing hormone (LH) surge. This work evaluated the anterior pituitary (AP) component of the H-P axis by determining the ability of perifused AP to release LH following sustained but pulsatile LHRH stimulation. The normal dual discharge profile of LH was affected by age. The first hour of the response, unaffected by cycloheximide, was similar in 5–6 month-old (mature), 12–13-month-old (declining litters) and 16–18-month-old (irregularly cycling) mice. The remaining protein synthesis-dependent part of the response was reduced in the 16–18 and 22–24-month-old (anestrus) mice. The role of estradiol (E2) in AP aging was further tested as AP from ovariectomized (OVXed) mice, deprived of E2 since puberty, responded as well as the mature proestrous group. In contrast, aged mice subjected to long-term E2 exposure (cycling or OVXed plus E2 replacement) failed to produce the dual response pattern. Since alterations in LH response occurred during the protein-dependent phase, synthetic processes that involve packaging and transport of stored LH, or the production of new LH, may be affected by age. Furthermore, E2 is a major factor in altering LH function and appears to act before middle age.

The association of the pre-ovulatory surge of LH and conception in beef cattle

This study addressed the question of whether a delayed pre-ovulatory luteinizing hormone (LH) surge in respect to the onset of estrus was responsible for lack of conception in beef cows ( Bos taurus). Thirty beef cows were cannulated for collection of blood samples at 2-h intervals for 30 h following the onset of standing estrus. Cows were artificially inseminated 12 h after the onset of estrus and palpated per rectum 40–60 days post-breeding to determine pregnancy ( n=17) or non-pregnancy ( n=13). All 30 cows had a pre-ovulatory surge of LH within first the 6 h after standing estrus. The length of standing estrus was not different between pregnant and non-pregnant cows (7.4±0.9 h vs. 6.0±0.7 h). The length of standing estrus (ranging from 2 to 16 h) was not related to the timing of the surge of LH. These results indicate that a delayed pre-ovulatory LH surge is not a major factor in failure to conceive, and support the concept that time of insemination should be based on the onset and not length of standing estrus.

Ovarian follicular dynamics and concentrations of oestradiol-17 beta, progesterone, luteinizing hormone and follicle stimulating hormone during the periovulatory phase of the oestrous cycle in the cow

Changes in the plasma concentration of oestradiol-17 beta, progesterone, luteinizing hormone (LH) and follicle stimulating hormone (FSH) were characterized during the transition from the luteal to the follicular phase, the periovulatory period and the early luteal phase in five cycling cows. The pattern of growth and the regression of follicles and corpora lutea in the ovary of the same animals were also assessed by daily ultrasonographic examinations. Two waves of follicular growth (ovulatory and non-ovulatory) occurred in all animals. The ovulatory follicular wave started from 4 days before the preovulatory surges of LH and FSH and the wave of next growth of a dominant follicle (non-ovulatory follicle) started within one day after ovulation. Changes in plasma levels of oestradiol-17 beta correlated well with the growth of both ovulatory and non-ovulatory dominant follicles. Suppression of FSH concentration during the follicular phase was inversely related to the increase in plasma concentration of oestradiol-17 beta. These results suggest that, in the cow, ovulatory dominant follicles suppress FSH secretion by increasing the concentration of oestradiol-17 beta (and probably also inhibin) during the follicular phase.

Quantification and classification of pregnancy wastage in 5-day cyclic young through middle-aged rats

Fecundity was quantified in 4 to 11-month old rats with regular 5-day cycles. On the day following mating, less than 5% luteinized unruptured follicles occurred in either 4-month old or 11-month old rats; ovulation rate and also fertilization rate were similar in these young and middle-aged rats. At day 19 of pregnancy embryonic and fetal mortalities were quantified. Pregnancy wastage increased gradually with age; it was already significant in 7-month old rats. In rats of 9 months old it was about 30% and in 11-month old rats it was greater than 65%. Pregnancy wastage was mainly due to preimplantation and early-postimplantation mortality. Pregnancy wastage did not decrease when the pre-ovulatory surge of luteinizing hormone (LH) was reinforced and timed, by injection of the LH-releasing hormone analogue Ovalyse at noon on the day of prooestrus. The results corroborate the idea that pregnancy wastage in middle-aged rats with regular cycles is caused by an age-related reduction of the viability of ovulating eggs.

Physiologically important role for central oxytocin in the preovulatory release of luteinizing hormone

Recently, our laboratory has provided evidence for a physiologically relevant stimulatory influence of oxytocin (OXY) on the preovulatory luteinizing hormone (LH) surge in cycling female rats. The present study evaluated whether this stimulatory effect of OXY on LH release is exerted at a central or peripheral site of action by comparing the ability of peripheral (intravenous) or central (intracerebroventricular) administration of OXY antisera to influence the preovulatory LH surge. The peripheral injection of a very large dose of OXY antisera (0.8 ml) caused a slight attenuation of the early stages of the LH surge. In contrast, the central administration of 5 μl of OXY antisera completely abolished the preovulatory LH surge. The data support the hypothesis that OXY exerts a physiologically important stimulatory influence on the preovulatory LH surge which is mediated primarily at a central site of action.

Effect of 20α-dihydroxyprogesterone on the estrogen-induced bioactive luteinizing hormone surge in ovariectomized monkeys

Progestins facilitate estrogen induction of the preovulatory luteinizing hormone (LH) surge. A biologically weaker progestin, 20 alpha-dihydroxyprogesterone, is reportedly increased at midcycle, but the importance of 20 alpha-dihydroxyprogesterone in regulating LH release has not been systematically studied. We therefore evaluated the effect of 2 doses of 20 alpha-dihydroxyprogesterone on the bioactive LH surge onset in ovariectomized monkeys. Chronically cannulated monkeys (n = 10) received estradiol (E2) benzoate (50 micrograms/kg) with and without subsequent 20 alpha-dihydroxyprogesterone injections (50 micrograms/kg, n = 3 and 100 micrograms/kg, n = 3). Blood was collected every 15 minutes over preselected intervals and the plasma assayed for bioactive LH and gonadal steroids. Low-dose 20 alpha-dihydroxyprogesterone administration resulted in periovulatory peripheral 20 alpha-dihydroxyprogesterone levels and did not advance the bioactive LH surge onset. In contrast, high-dose 20 alpha-dihydroxyprogesterone resulted in supraphysiological 20 alpha-dihydroxyprogesterone levels, slight increases in progesterone levels, and significant advancement of the surge over E2 benzoate treatment alone. We conclude that periovulatory levels of 20 alpha-dihydroxyprogesterone do not play a role in modulating the estrogen-induced bioactive LH surge.

Association between the preovulatory luteinizing hormone surge and the early ultrasonographic detection of pregnancy and fetal heartbeats in beagle dogs

The size of the gestational sac and embryonic mass as well as the embryonic heartbeat were examined ultrasonographically from Day 16 to 25 of pregnancy in 15 beagle bitches, using a 7.5 MHz transducer. Results were more consistent when gestational age was based on the day of the preovulatory luteinizing hormone (LH) surge than on the day of first breeding. The gestational sac was first detected at 17 to 20 d after the LH surge, when it was 1 to 2 mm in diameter and 1 to 4 mm in length. The diameter and length of the gestational sac increased exponentially. At Day 25, the mean diameter was 8.2 ± 0.3 mm (7 to 9 mm) and mean length was 20.3 ± 1.1 mm (14 to 24 mm). Embryonic mass and heartbeat were first detected at 23 to 25 d after the LH surge. The embryonic heartbeat was detected on the day of or the day after detection of the embryonic mass, at which time the embryonic mass was 1 to 4 mm in length and was located at the periphery of the gestational sac.

Ovulatory responses to continuous administration of GnRH in nine-month-old prepubertal beef heifers

Continuous infusion of gonadotrophin-releasing hormone (GnRH) will consistently induce ovulation in several species, but has had limited success in post-partum acyclic cattle. We suggest that this treatment is only effective when anovulation is due solely to a lack of gonadotrophic stimulation of the ovary, rather than to a hypothalamic-pituitary block. Therefore, we have attempted to induce ovulation in 9-month-old prepubertal heifers, an experimental model in which lack of ovarian stimulation is likely to be the only limitation to oestrous cycles. GnRH was administered either at a controlled rate (5 μg/h) via osmotic minipumps (inserted s.c.), or at a less predictable rate by s.c. implantation of a biodegradable implant (40 μg GnRH/implant) previously shown to be effective in sheep. The mean magnitude of the maximum luteinizing hormone (LH) concentration in response to GnRH treatment was significantly higher in heifers treated with 24 GnRH implants than in those given 8 implants or osmotic minipumps (18.02 ± 1.23 ng/ml, 11.55 ± 2.37 ng/ml and 8.6 ± 2.29 ng/ml, respectively). In addition, plasma LH concentrations reached preovulatory surge magnitude within 4 h of implantation in 5 5 , 1 6 and 0 5 animals in these treatments groups. Conversely, an endogenous preovulatory LH surge was recorded in 0 5 , 3 6 and 5 5 heifers treated with 24 implants, 8 implants or osmotic minipumps. The consistent ovulatory response recorded in those heifers treated with osmotic minipumps demonstrates the importance of administering GnRH at a constant and controlled rate.

Estrous Cycle Variations in Gonadotropin‐Releasing Hormone, Substance P and Beta‐Endorphin Contents in the Median Eminence, the Arcuate Nucleus and the Medial Preoptic Nucleus in the Rat: A Detailed Analysis of Proestrus Changes

Abstract The concentrations of gonadotropin-releasing hormone, substance P and beta-endorphin were measured in the median eminence, the arcuate nucleus and the medial preoptic nucleus of 4-day cycling female rats. Very well marked estrous cycle-related fluctuations were registered for these neuropeptides in these areas. The largest variations in concentrations of peptide levels were observed in the median eminence. Substance P concentration was highest throughout the day of proestrus as compared to the three others days of the cycle. At the time of the preovulatory luteinizing hormone surge, on the afternoon of proestrus, there was a marked increase in gonadotropin-releasing hormone concentration and a marked decrease in beta-endorphin concentration.

Endocrine responses in the llama to copulation

Nine adult female llamas were used to determine the time course for secretion of luteinizing hormone (LH) and estradiol-17beta (E(2)) following a single copulation (average 18 min), and progesterone (P(4)) during the development of the subsequent luteal phase. Heparinized blood samples were obtained through an indwelling jugular cannula at 15-min intervals for up to 24 h following copulation and then once daily for up to 10 d. Luteinizing hormone, assayed by radioimmunoassay (RIA) using a monoclonal antibody 518B7 against the beta subunit of bovine LH, was determined at 15 min intervals for 24 h following copulation. Estradiol-17beta was determined by RIA at 4-h intervals following copulation, then daily, while P(4) values were determined daily by enzyme immunoassay. A significant increase in LH concentration was observed by 15 min after the onset of copulation, with the peak of the preovulatory surge of LH occurring at 2 h; values were basal by 7 h after copulation. Estradiol-17beta values, unchanged through 18 h after copulation, tended to decline at 22 h (24 h, P<0.10) and were significantly lower than 18 h values by 48 h (P<0.05) after copulation. The first significant P(4) increase occurred at 3 d after copulation, with values increasing through 10 d. The LH surge observed subsequent to copulation is consonant with the llama being an induced ovulator.