Pituitary Luteinizing Hormone Levels Research Articles

Is disease flare a problem?

When given for the first time to previously untreated patients with advanced prostate cancer, luteinizing hormone-releasing hormone (LHRH) analogs induce a transient rise in pituitary luteinizing hormone levels. As a consequence of this increase of LH, there is, within the first 2 to 3 days, a surge of testosterone, which can cause an exacerbation of the symptoms. First reports concerning this flare have been anecdotal, and in most studies, flare is reported with an incidence of 4-33%. This variance is due mainly to the confusion about the definition of the flare phenomenon. No distinctions have been made between clinical flare, with its manifestations of subjective or objective aggravation of cancer related symptoms, and the biochemical flare that results of the LHRH analog administration and that occurs in a majority of patients and is characterized by increases in testosterone, prostatic acid phosphatase, and prostate specific antigen. As the possible interference of the flare phenomenon on the ultimate aftermath of the patient's response to therapy is not yet known, it seems mandatory that flare prevention should be carried out whenever LHRH analogs are prescribed in monotherapy.

Effect of infundibular lesions on GnRH and LH release in the frog, Rana temporaria, during hibernation

The effects of lesions and deafferentations on accelerated hibernatory ovulation and on the concentration of GnRH in blood collected from the hypothalamo-pituitary area along with the blood and pituitary levels of luteinizing hormone (LH) in hibernating female frogs, Rana temporaria, were studied. The lesions to the caudal portion of the nucleus infundibularis ventralis resulted in: (1) an elevation in GnRH concentration in blood collected from the hypothalamo-pituitary area ( x ± SEM = 51 ± 9.5 and 100 ± 15 pg/ml in control and lesioned females, respectively), (2) an increase in plasma LH (from 11 ± 1.3 to 14.7 ± 2.5 ng/ml in controls to 73.1 ± 12.0 and 74.2 ± 15.8 ng/ml in lesioned females at 3 and 7 days), and (3) accelerated ovulations whose onset advanced as the hibernation season progressed. Histological and immunohistochemical analyses of the brains showed that GnRH seems to be stored during hibernation in the median eminence and suggested that the complete disruption of projections to the median eminence is prerequisite for accelerated ovulation. The influence of the hypothalamic inhibitory action on the release of GnRH and the way in which GnRH is transferred to the pituitary gland are discussed.

Central administration of neuropeptide Y induces precocious puberty in female rats.

To examine the role of neuropeptide Y (NPY) in the regulation of puberty, the effects of various doses of NPY or saline injected into the 3rd ventricle of peripubertal female rats were determined. A single injection of NPY at a dose range of 10-20 micrograms into the 3rd ventricle of 30-day-old female rats advanced the time of vaginal opening (VO) and first ovulation by 4 days, as compared to saline-treated controls. Ova number and ovarian, adrenal and uterine weights at first estrous were similar in all animals regardless of treatment. The NPY-treated precocious animals showed lower body weight than did the VO-matched controls, but showed similar body weight as that of age-matched controls. Determination of plasma luteinizing hormone (LH) levels in the peripubertal female rats revealed that plasma LH was increased transiently immediately after NPY administration. Also, NPY-treated rats which had precocious puberty showed elevated pituitary and plasma LH levels on the day of VO. These results suggest that NPY can activate the hypothalamic-pituitary-gonadal axis to precociously initiate puberty.

Neuroendocrine Function in Transgenic Male Mice with Human Growth Hormone Expression

The neuroendocrine effects of human growth hormone (hGH) secretion were studied in adult male mice into which an hGH gene fused with mouse metallothionein 1 (mMT-1) promoter had been introduced. Intact transgenic mice had significantly greater plasma luteinizing hormone (LH) levels than did normal littermate controls. Castration increased LH levels in normal mice but was without effect on plasma LH levels in the transgenic mice. In vitro LH secretion and pituitary LH content were higher in the intact transgenic mice than in intact controls, while there was no significant difference in pituitary LH levels and in vitro LH secretion between the 2 groups of castrate animals. Intact transgenic mice exhibited a greater median eminence (ME) norepinephrine (NE) turnover than control animals, but ME NE turnover did not increase after castration in the transgenic animals as was the case in control mice. Castrate mice expressing the hGH gene had plasma levels of prolactin (PRL) similar to those seen in castrate controls, which was unexpected based on a previous study showing greatly attenuated PRL levels in intact hGH mice when compared to intact controls from the same line. Dopamine (DA) turnover in the ME was not significantly affected by the presence of the hGH gene, suggesting that the difference in plasma PRL levels between normal and transgenic mice is mediated through changes in PRL-regulating factors other than DA. In conclusion, the expression of the mMT-1/hGH hybrid gene in male mice leads to major alterations in LH secretion and lesser changes in PRL secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Lithium suppresses ovariectomy-induced surges in plasma gonadotropins in rats

We have reported earlier that administration of lithium, the widely-used drug for the treatment of acute mania and prophylaxis of recurrent manic-depressive bipolar disorders, leads to a disruption of estrous cycle and a significant suppression of the proestrous surge of luteinizing hormone (LH) in a number of laboratory rodents. In this report we have examined the effects of this antimanic drug on plasma and pituitary levels of LH and follicle stimulating hormone (FSH) in rats following ovariectomy (OVX), an altered endocrine state in which the levels of serum LH and FSH are highly andchronically elevated. Adult OVX rats, maintained under standardized laboratory conditions (LD 12: 12; white lights on at 06.00 h, CST) were injected (ip) with lithium, 40 days post-operation, at a dosage of 3.0 and 2.0 mEq/Kg b. wt. for 3 and 7 days respectively (twice daily at 08.00 and 16.00 h). Control OVX rats received nothing or saline injections, whereas an intact control (C) received no surgical manipulation or drug injections of any kind. As expected, the levels of plasma LH and FSH in OVX (only) group showed nearly 6-fold and 75-fold increase respectively compared to those in C. Lithium injections in OVX rats for 3 and 7 days resulted in a significant reduction in plasma LH (P<.005 and P<.02 respectively) and FSH (P<.001) levels when compared with those in the OVX control groups. Lithium also led to a significant reduction in the levels of pituitary LH after both 3 (P<.02) and 7 days (P<.02), but the levels of pituitary FSH remained unchanged. These results suggest that the pituitary gonadotropes constitute a definitive target for lithium's action, either directly or via the hypothalamus.

Photoperiodic control of puberty in the red-legged partridge ( Alectoris graeca chukar)

The photoperiodic control of puberty in birds was studied using hybrid red-legged partridges ( Alectoris graeca chukar). In the wild partridges first breed at about 10 months of age. Males and females were held from hatch on short (8L: 16D) daylengths and then at 3, 4 1 2 , 6, 7 1 2 , 9, and 12 weeks of age groups were transferred to long daylengths (20L:4D) for 3 weeks to test for the photoperiodic induction of gonadal growth. Control groups were maintained on either the short or long daylengths and killed at 3, 9, and 12 weeks of age. Gonadal growth did not occur in either of the control groups, nor in any of the birds photostimulated prior to 6 weeks of age. However, in birds photostimulated after this point gonadal size was increased in a graded manner, this growth being reflected in the plasma and pituitary levels of luteinizing hormone. Similar long-day responses which increased with age were observed in plasma and pituitary levels of prolactin (PRL). In the control birds there was a steady decline with time in plasma PRL, with no change being seen in pituitary PRL. Hypothalamic gonadotrophin-releasing hormone content did not change in birds maintained on 8L-:161) or 20L:4D and was only elevated by photostimulation in the females. Plasma testosterone remained low in all the males except for those transferred to 20L-:4D at 12 weeks. Testicular testosterone content remained low in control birds but was elevated by photostimulation at 6 and 12 weeks of age. Plasma progesterone levels increased with age in control birds and were only elevated by photostimulation at 3 and 4 1 2 weeks of age. These data show juvenile partridges to be completely unresponsive to photoperiodic stimulation and in a condition very similar if not identical to that seen in adult refreactoriness. As in the adults, this refractoriness is progressively dissipated by exposure to short days so that by 9–12 weeks of age the partridges are able to respond maximally to long-day stimulation.

Lithium: Evidence for reduction in circulating testosterone levels in mice following chronic administration

Lithium, the widely-used antipsychotic drug, is known to exert adverse effects on a number of endocrine organs. In the present investigations, the effects of chronic lithium administration on circulating levels of testosterone and plasma and pituitary levels of luteinizing hormone (LH) were evaluated in order to examine whether or not the pituitary-gonadal axis is a probable target of lithium action. Adult male C57BL/6 mice, maintained on a fixed photoperiodism (LD 14:10), were administered lithium orally, by being fed on a specially prepared chow containing 0.4% lithium chloride for 15 or 30 days, while their matched controls were maintained on standard laboratory chow. At the termination of the respective experimental schedules, the animals were decapitated, their blood collected, and plasma was separated and stored frozen. Pituitaries were quickly removed, weighed, homogenized, centriguged and their supernatants were stored frozen. Testosterone in plasma and LH in pituitary and plasma were quantitated by standard RIA methods. Plasma Li concentration was determined by using flame photometric methods. A significant suppression in testosterone levels was noted after both 15 (p < .01) and 30 (p < .05) days of lithium treatment, but both pituitary and plasma LH levels remained unchanged at both the periods. It is, therefore, suggested that lithium exerts its effect directly at the level of the Leydig cells rather than through the pituitary-gonadal axis. Since the noted lithium-induced reduction of testosterone was manifested when the plasma lithium levels were within (or around) the therapeutic range, these results may have important clinical implications.

Transplanted Gonadotropin-Releasing Hormone Neurons Promote Pulsatile Luteinizing Hormone Secretion in Congenitally Hypogonadal (hpg) Male Mice

Congenitally hypogonadal (hpg) male mice are unable to synthesize biologically active gonadotropin-releasing hormone (GnRH). Implantation of normal fetal preoptic area tissue containing GnRH neurons into the third ventricle of adult hpg males significantly elevates pituitary levels of luteinizing hormone (LH) and corrects their hypogonadism. In all responding animals, immunoreactive GnRH neurons within the transplant innervate the median eminence of the host. To assess whether gonadal recovery in hpg hosts results from pulsatile secretion of GnRH from grafted neurons, we compared the pattern of variation in plasma LH levels in 19 hpg graft recipients with testicular growth to that of 10 normal adult mice. All animals were castrated prior to receiving an indwelling catheter in the jugular vein. Sequential blood samples were collected (t = 10 min) and assayed for LH. Pulsatile LH secretion was seen in 11 of 19 hpg hosts and in all control mice. While there was great variability between individual animals, measures of baseline LH, LH pulse amplitude and duration, interpulse interval, and LH pulse frequency revealed no difference between hpg graft recipients and normal castrates in their LH pulse pattern. Immunocytochemical analysis of the brain in hpg hosts suggested no correlation between any parameter of pulse activity and individual differences in GnRH cell number or GnRH fiber outgrowth into the median eminence. Sources of variation in LH secretion among graft recipients, and between hpg hosts and normal mice, are discussed. We suggest that transplanted GnRH neurons are capable of integration into a GnRH 'pulse generator' which can support a near-normal pattern of pulsatile LH secretion, leading to testicular growth and steroid production.

Effects of lithium on the pituitary-gonadal axis in the rat: Evidence for dose-dependent changes in plasma gonadotropin and testosterone levels

The purpose of the present study was to examine the effects of lithium, a drug which is now used rather widely in the treatment of acute mania and the prophylaxis of manic-depressive bipolar disorders, on the pituitary-gonadal function in the laboratory rat. Sexually adult male rats, maintained under standardized laboratory conditions (LD 14: 10; lights on at 06:00 h, CST),were injected (ip) with lithium chloride both acutely for 1 day and chronically for 5 days, and by utilizing a low and high dose. For the low dose, lithium was injected twice daily (at 10:00 and 15:00 h) at 2.5 meg/Kg for 1 and 5 days, whereas in the high dose groups, also receiving lithium twice daily and at the same hours, the dosages were 5 meg/Kg for 1 day and 3.5 meg/Kg for 5 days. Animals were sacrificed 4 hours after the last lithium (or saline) injections. Plasma and pituitary levels of luteinizing hormone (LH) and follicle stimulating hormone (FSH), and plasma levels of testosterone (T) were measured by radioimunoassay (RIA). The administration of the low dose led to a significantly higher (P <0.001) plasma FSH, but unaltered plasma LH, levels after 5 days. In contrast, the high dose lithium led to significant suppressions of plasma LH (P <0.02; on day 5) and FSH (P <0.001; on both day 1 and 5) levels. The levels of plasma T also showed a significant reduction following the low dose (P <0.02; on day 5), as well as the high dose lithium treatment, as evident after both 1 (P <0.02) and 5 (P <0.02) days. Regardless of the dosage, or the duration of treatment, pituitary gonadotropin levels remained unaltered following lithium. The results of our present experiments suggest that lithium administration, either acutely or on a chronic basis, might be associated with significant adverse effects on the pituitary-testicular axis. Furthermore, since some of the hormonal changes were evident when plasma lithium concentration was within the therapeutic range, our data may have potential clinical implications.

The effect of alcohol on quantitative and qualitative changes in luteinizing hormone (LH) in the female rat.

In order to study the impact of short-term ethanol feeding on pituitary luteinizing hormone (LH) levels, LH release and hormone microheterogeneity, 28 female oophorectomized rats were fed either a diet containing 36% ethanol or an isocaloric diet without ethanol for 16 days. On the 14th day of the experiment, all rats were given 50 micrograms of estradiol subcutaneously to provide a uniform steroid milieu and to induce an LH surge, and sacrificed 48 hours later. Total levels of pituitary and serum immunoreactive LH were identical between the ethanol and control groups. However, when pituitary LH was analyzed by isoelectric focusing on agarose gel a distinct difference was found. Four distinct immunoreactive peaks at pI's 3.50-4.55, 5.20-6.50, 7.00-7.50, 8.15-9.30 were noted. Ethanol-treated animals exhibited an 8-fold increase in the acidic LH subspecies in the pI range 3.50-4.55 and similarly more LH was determined at pI 7.00-7.50 compared to controls (P less than 0.01). More LH was found at the basic pI's from control pituitaries, compared to LH from the ethanol-exposed animals (P less than 0.01). In summary, short-term ethanol feeding in the female rats does not cause quantitative changes in pituitary LH levels or in the estrogen-induced LH surge. However, qualitative changes are present in isoelectric focusing patterns. A significant shift to the more acidic species of the pituitary LH isohormone was noted for ethanol-fed animals as compared to controls. These qualitative changes in the LH molecule, seen before quantitative changes could be appreciated, may help to explain why excessive exposure to alcohol results in reproductive impairments both in rats and in humans.

Selective release of luteinizing hormone by 3 alpha-hydroxy-5 alpha-pregnan-20-one in immature ovariectomized estrogen-primed rats.

This study investigated the role of 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-THP) in the modulation of gonadotropin secretion using the immature ovariectomized (OVX) rat primed with a low dose of estradiol. A treatment regimen of either 0.2 or 0.4 mg/kg of 3 alpha,5 alpha-THP given in conjunction with estradiol for 4 days significantly increased levels of serum luteinizing hormone (LH) but had no effect on serum follicle-stimulating hormone (FSH). Estrogen-primed rats receiving a single injection of 3 alpha,5 alpha-THP at 0930 h showed an increase in serum and pituitary LH levels at 1200 h and 1500 h. At 1800 h, only pituitary levels of LH remained significantly higher than controls. An injection of 3 alpha,5 alpha-THP at 1230 h in estrogen-primed rats resulted in enhanced levels of pituitary LH at 1500 h and elevated levels of both serum and pituitary LH at 1800 h. When 3 alpha,5 alpha-THP was given at 0930 h and 1230 h, elevated serum levels of LH were maintained for over 6 h. The administration of pentobarbital (Pb) 30 min after an injection of 3 alpha,5 alpha-THP at 0930 h or 1230 h prevented the increases in serum LH at 1200 h, 1500 h or 1800 h. This suggests that LH-releasing hormone (LHRH) is involved in mediating the LH response by 3 alpha,5 alpha-THP. There was no change in the sensitivity of the pituitary to LHRH following 3 alpha,5 alpha-THP treatment, indicating the absence of a pituitary effect of this steroid.

Transection of the spermatic vessels (Bevan's technique): an experimental study.

Intracanalicular or intra-abdominal undescended testicles often present a difficult surgical problem, due to a vascular pedicle shortening that prohibits scrotal placement with a 1 stage orchiopexy. The division of spermatic vessels to properly place the testis in the scrotum solves the problem of the shortening of the vessels, but raises the question of whether the endocrine function and the potential for fertility are preserved. In the present study transection of spermatic vessels before arterial branch-off was done intra-abdominally in rats. Angiographic, patho-anatomic and hormonal evaluations have been made. Revascularization of the testicular artery through the vasal artery has occurred in all cases. The anastomotic circuit, however, does not always have the same functional value. The spermatogenic alterations suggest that fertility can be impaired in a significant percentage of cases. Spermatogenesis was quantified taking into account the mean tubular diameter and the situation of seminiferous epithelium, expressed as the percentage of tubules containing well-developed spermatogenesis. Plasma testosterone levels are in a low range. However, Leydig cell functional response to human chorionic gonadotropin stimulation is maintained in every case and so are plasma and pituitary luteinizing hormone levels, suggesting that plasma testosterone levels are high enough to maintain an adequate hormonal activity.

Hypothalamic gonadotropin-releasing hormone and pituitary luteinizing hormone contents in the male vole (Microtus agrestis) in the field during the breeding and nonbreeding seasons

Pituitary luteinizing hormone (LH) and hypothalamic gonadotropin-releasing hormone (GnRH) contents were measured in male voles trapped in the field over a period of 5 years. Pituitary LH contents were positively correlated with testicular and seminal vesicle weights but hypothalamic GnRH contents showed no correlation with pituitary LH contents or organ weights. Pituitary LH contents and testicular and seminal vesicle weights increased between January to April, reached peak values during May to July, and fell from August to December. Hypothalamic GnRH contents were high for most of the year with low values during December and January. During July, August, and September two populations of males were studied; a group with large active testes and high pituitary LH levels and a group with small testes and low pituitary LH levels. With the exception of the group in July of animals with small testes, all males, both sexually active and inhibited, had high hypothalamic GnRH levels.

Lithium-induced changes in the plasma and pituitary levels of luteinizing hormone, follicle stimulating hormone and prolactin in rats

Adult male Sprague-Dawley rats, maintained under a controlled photoperiod of LD 14:10 (white lights on at 06:00 h, CST), were injected with lithium chloride and changes in the levels of plasma and pituitary homogenates of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (PRL) were examined to evaluate the effects of this anti-manic drug on reproductive function. Two groups of rats were injected with lithium chloride intraperitoneally, twice daily at 09:00 and 16:00 h, for 2 and 7 days at a dosage of 2.5 meg/Kg body weight. Plasma and pituitary levels of LH, FSH and PRL were measured by radioimmunoassay. Plasma levels of LH were significantly (P<0.05) increased after 2 days of lithium treatment. In contrast, a significant (P<0.005) reduction in plasma levels of LH was evident when lithium injections were continued for 7 days. The plasma levels of FSH remained unaffected by lithium treatment by either time period. Lithium administered for 2 days did not bring about any significant alteration in the plasma levels of PRL, although there was a significant (P<0.002) reduction in plasma PRL levels after 7 days treatment. The concentrations of pituitary LH, FSH and PRL remained unchanged after 2 and 7 days of lithium treatment.

Effects of a bovine pineal peptidic fraction (E5) on plasma and pituitary levels of LH, FSH ans prolactin.

Pituitary levels of prolactin were significantly diminished after 5 s.c. injections spaced 12 h apart of 5 μg of a partially purified bovine pineal peptide fraction (E5) in both intact and castrated rats. E5 treatment did not significantly affect the castration-induced changes in plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH) or pituitary LH but did partially block the pituitary fall in FSH in castrated animals.

Effects of advancing age on the hypothalamic-pituitary-ovarian axis of the female white-footed mouse (Peromyscus leucopus).

Peromyscus leucopus, with an average lifespan of 48 months, showed unchanged levels of serum luteinizing hormone (LH), estradiol, progesterone, and pituitary LH and prolactin, between the ages of 12 and 48 months. Hypothalamic LH-releasing hormone (LHRH), norepinephrine and dopamine also remained unchanged with advancing age. Ovarian and uterine weight decreased with age, although the changes in uterine weight were not statistically significant. These data indicate that the hypothalamic-pituitary-ovarian axis remains intact with increasing age, accounting for the maintenance of fertility in these animals. The lack of significant changes in these parameters is in very marked contrast to those in the aging laboratory mouse and rat, which show derangements in their reproductive systems midway through their lifespans.

Changes in the pituitary and plasma LH, plasma and follicular progesterone and estradiol, and plasma testosterone and estrone concentrations during the ovulatory cycle of the quail ( Coturnix coturnix japonica)

Luteinizing hormone (LH) and sex steroid hormones in the pituitary, plasma, and hierarchical follicles of the quail maintained under 14L:10D condition were measured during the ovulatory cycle at 2-hr intervals using radioimmunoassay. Pituitary levels of LH during the ovulatory cycle showed two peaks which occurred 16 and 10 hr before ovulation. A significant decrease in LH concentration was observed from 10 to 6 hr before ovulation, shortly after the onset of light (dawn). Plasma LH started to increase from 10 hr before ovulation, and reached a peak 6 hr before ovulation. Plasma progesterone also started to increase shortly after dawn and reached a peak 4 hr before ovulation. A single peak of plasma testosterone was observed 6 hr before ovulation. Two peaks of plasma estradiol concentration were observed at 22 and 6 hr before ovulation. A similar tendency of the change in plasma concentration of estrone to that of estradiol was observed during the ovulatory cycle. In the largest follicle, the progesterone content started to increase from 10 hr before ovulation, reached a peak 2 hr before ovulation and decreased abruptly after the ovulation. The contents of the second to fourth largest follicles were low. These findings strongly suggest that progesterone in the largest follicle is a main cause of ovulation by producing enzymes which act to break the follicle membrane. The estradiol contents of the third and fourth largest follicles were high, and showed the peak at 18 hr before ovulation in the third largest one and at 8 hr before ovulation in the fourth largest one. The contents of the largest, ruptured, and the second largest follicles were low compared with the third or fourth largest one. These results suggest that the onset of light may be a signal for the increases of plasma LH and progesterone and follicular progesterone which are responsible for the induction of ovulation in the quail.

Luteinizing Hormone Secretion in Hemidecorticate Female Rats Under Several Experimental Conditions

Luteinizing hormone (LH) secretion by the anterior pituitary gland was investigated in hemidecorticate (HD) rats under several conditions. Higher plasma and lower pituitary LH levels were observed in HD rats in the afternoon of proestrus, after unilateral ovariectomy, and at the 7th and 14th days after bilateral castration. The ovarian hypertrophy observed in HD rats did not differ from control. When hormonal substitutive therapy was started 3 days after bilateral ovariectomy, larger doses of estradiol benzoate (EB) were required to reduce plasma LH levels in control than in HD animals. The increase in pituitary weight was, however, significantly higher in the HD castrated group. When substitutive therapy started at the 7th and 14th days after castration no significant differences between the 2 experimental groups were observed. The results of substitutive therapy suggest that hemidecortication induces an increase in the sensitivity of hypothalamic-pituitary axis to estrogen negative feedback. This change in sensitivity is clearly seen in HD rats during the first few days after castration. These results suggest that hemidecortication may release the hypothalamus from inhibitory influences coming from rhinencephalic or cortical structures, thus rendering the preoptic-suprachiasmatic complex (POA-SCH) more sensitive to LH-releasing mechanisms.

Dopamine sensitivity of hypothalamic arcuate neurons and their role in the regulation of pituitary gonatropic function

The effect of microiontophoretic injection of dopamine (DA) into the arcuate region of the hypothalamus on single unit activity and the plasma and pituitary levels of luteinizing hormone (LH) was investigated in rats in various stages of the estrous cycle. No significant differences in the number of neurons responding by activation or inhibition or not responding to DA could be observed in the course of the estrous cycle. However, in the first half of the day of proestrus (P) a significant increase in the number of neurons responding by activation to microiontophoretic injection of DA was observed. In all stages of the estrous cycle except the second half of P the activation response of the neurons was coupled with elevation of the plasma LH level. A significant rise in the LH level in the pituitary in response to microiontophoretic injection of DA into the hypothalamic arcuate region was observed only in stage diestrus 2.

Restoration of fertility in light-deprived female hamsters by chronic melatonin treatment

Blinding young adult female hamsters was followed by functional involution of the ovaries and uteri and by the cessation of cyclic vaginal phenomena. Light deprivation was also accompanied by elevated plasma and pituitary levels of luteinizing hormone and depressed levels of prolactin in both the blood and the pituitary gland. Only one of 15 blinded hamsters became pregnant when they were exposed to fertile males for 30 days. Both pinealectomy or chronic melatonin treatment (1 mg melatonin implanted subcutaneously per week in beeswax) prevented the changes in the reproductive organs and in pituitary hormone levels attendant on light-deprivation. Both treatment also returned vaginal cycles to normal and restored plasma prolactin titers. Unlike hamsters that were blinded only, light deprived hamsters that were either pinealectomized or melatonin treated were capable of reproducing when they were caged with fertile males. The reproductive capability (i.e., percent of animals that become pregnant and the sizes of their litters) of these animals was equivalent to that of the untreated control hamsters. This is the first report that chronic melatonin treatment restores fertility in blinded female hamsters.