Luteinizing hormone response to different doses of synthetic gonadotropin-releasing hormone during early and late lactation in primiparous sows

The object of this investigation was to study the luteinizing hormone (LH) response to different doses of synthetic exogenous gonadotropin-releasing hormone (GnRH) during early (days 7–15) and late (days 20–28) lactation in primiparous sows. Five crossbred primiparous lactating sows were used in this study. The lactation period was 35 days. Five doses of GnRH, 0, 2.5, 5.0, 10.0 and 20.0μg were administered via a jugular vein catheter every second day in a latin square design during both the early and late lactation period. Daily (9.00 a.m.) blood samples were taken for oestradiol–17β and progesterone analysis. Frequent blood samples were taken before and after GnRH treatment for LH analysis. The total LH response was measured from post-treatment samples as the area under the curve above the base level, obtained from pre-treatment samples.No elevation of plasma oestradiol–17β or plasma progesterone occurred during lactation. The LH response increased with increasing doses of GnRH during early as well as late lactation. The total LH response during late lactation was a little greater than during early lactation, but the difference was not significant (P=0.055). There was no effect of the time sequence of treatment within phase of lactation.     

Hormonal changes during the perinatal period: Serum testosterone,some of its precursors,and FSH and prolactin in preterm and fullterm male infant cord blood and during the first week of life

To investigate fetal regulation of the endocrine testis during the third trimester of gestation, pregnenolone, progesterone, 17-hydroxyprogesterone, androstenedione, testosterone, FSH and prolactin concentrations were measured in the umbilical circulation of 31–35 preterm (27–37 weeks) and 18–19 fullterm (39–42 weeks) male infants, and postnatally until 5 days of age in 27–39 fullterm male infants. 17-hydroxyprogesterone and prolactin concentrations increased significantly (P < 0.001) between 27–37 weeks of gestation; the other hormones measured were unchanged. The levels of progesterone in preterm infants, and prenenolone, progesterone and 17-hydroxyprogesterone in the cord vein of fullterm infants were significantly (P < 0.001–0.05) higher than those in the cord artery. Androstenedione concentrations were similar in the cord artery and vein, and decreased less than pregnenolone, progesterone and 17-hydroxyprogesterone after birth, reflecting major androstenedione production in the fetus. Testosterone concentrations were higher (P < 0.01–0.05) in the cord artery than in the vein, both in preterm and fullterm infants, showing the main site of testosterone production to be the fetal compartment. Postnatally, testosterone increased clearly from concentrations of 0.25 ± 0.05 (SE) mg/ml in the cord artery and 0.10 ± 0.01 in the cord vein to 0.94 ± 0.14 ng/ml in the peripheral vein on the first postnatal day, and decreased thereafter clearly between 3–5 days. FSH did not change during the first 5 postnatal days. Concentrations of all the other hormones measured decreased significnatly after birth.It is concluded from the cord blood hormone levels of infants born between 27–42 weeks of gestation that: (1) The third trimester of gestation represents a stable phase of endocrine development with relatively small changes in circulating hormone levels; (2) Both the placenta and the fetus seem to be able to produce androstenedione in the perinatal period; and (3) The initial increase in testosterone after birth is indicative of the inhibitory effect of placental steriods on testicular endocrine function during the last trimester of gestation.     

Comparative analysis of functions of cow and reindeer female reproductive organs: Progesterone content in vessels of reproductive organs

Progesterone content in blood from paired ovarian and uterine veins as well as from jugular veins of cows and reindeers was studied in the estrous cycle lutein phase and at the earlier stages of pregancy. In the both species, maximal progesterone concentration was detected in blood from vein of the ovary carrying corpus luteum (p < 0.001). In cows, a higher hormone concentration, as compared with jugular vein, has also been determined in vein of the uterus horn closest to ovary with corpus luteum (p < 0.01). In reindeers, blood from all studied vessels of reproductive organs had the progesterone concentration that was statistically significantly higher (p < 0.001) than that from jugular vein. In cows, progesterone concentration in blood from the ovarian vein was found to be higher when corpus luteum was located on the right ovary (p < 0.05) as compared with left-side corpus luteum location. No functional asymmetry of ovaries was revealed in reindeers. A possible role of mechanisms of the hormone local transport between ovary and uterus in adaptation of ruminants to reproduction under Nordic conditions is discussed.     

On the regulation and turnover of progesterone metabolites in rat uterus

Recently the in vitro progesterone metabolism was shown to be inhibited in uterine tissue by association of the hormone with binding components. However, a dissociation of progesterone would impair the protection of the steroid hormone caused by complex formation. In order to study this effect, the influence of time was investigated on the metabolism of progesterone. Progesterone metabolites were analysed quantitatively from the recovered material of uteri and nutrient media by thin layer chromatography (TLC) at various time invervals. After finishing the incubation with the labelled steroid, the amount of progesterone metabolites produced increased continuously in the tissue during the following hour when the uteri were kept in nutrient medium. This indicated that the dissociation of progesterone from a hormone protein complex led to the subsequent metabolism of the unbound hormone. However, the metabolism was reduced markedly by an increase of the protein content in uterine tissue and with it by an increase of progesterone binding proteins in uterine cytosol as determined by charcoal adsorption technique. Additionally, the amount of progesterone metabolites was found to be much higher in uterine tissue than that released into nutrient medium during the time interval studied. Therefore, uterine tissue concentrates progesterone metabolites, and a rapid turnover of these substances does not occur.     

Secretion of progesterone during long and short days of the estrous cycle in goats that are continuous breeders

This study was conducted 1) to determine if the secretion of progesterone, as an index of ovarian activity, during the estrous cycle of nonseasonal Shiba goats is affected by seasonal changes, and 2) to learn if the pulsatile secretion of ovarian progesterone can be estimated from samples obtained by cannulation into the caudal vena cava via the femoral vein. Progesterone concentrations in jugular venous plasma during the estrous cycle in spring (May) were similar to those in autumn (November). Plasma progesterone concentrations in the jugular vein and caudal vena cava monitored for 10 h on Day 12 of the estrous cycle in spring were similar to those in autumn. The mean concentration (21.9 to 28.9 ng/ml) and the pulse frequency (6.2 to 7.4 pulses/10 h) of plasma progesterone in the caudal vena cava during both seasons were 3.1- to 4.7-fold and 1.7- to 2.4-fold those in the jugular vein, respectively. The degree of change in the peak magnitude and the base-line concentration of progesterone were higher in the caudal vena cava than in the jugular vein. These results indicate that progesterone secretion during the estrous cycle in nonseasonal goats is not affected by seasonal changes, and suggest that the pulsatile secretion of ovarian progesterone can be evaluated better from samples obtained from the caudal vena cava, near where progesterone is released, than from those obtained from the jugular vein.     

Secretion of estradiol-17beta by porcine mammary gland of ovariectomized steroid-treated sows

Although the mammary gland of many species secretes estradiol (E(2)), nothing is known of E(2) secretion in the porcine gland. The present study was designed to investigate whether porcine mammary gland was a source of E(2), and to test the influence of individual and combined effects of exogenous progesterone and estradiol benzoate (EB) on the secretion of E(2). Immature crossbred gilts were ovariectomized at 7 months of age followed by 4 weeks later by steroid hormone replacement therapy to produce estradiol and progesterone (P(4)) blood concentrations similar to those observed during a normal estrous cycle. Arterial and venous blood plasma (from carotid artery and anterior mammary vein, respectively) were sampled for 2h at 10 min intervals. Plasma concentrations of progesterone, androstenedione (A(4)), testosterone (T), estrone (E(1)) and estradiol were determined by RIA. In all gilts treated with progesterone alone or in combination with EB, concentrations of P(4), A(4) and E(1) in blood collected from venous outflow were lower compared to concentrations in arterial blood, whereas concentrations of E(2) were higher in blood plasma from the anterior mammary vein compared to plasma from the carotid artery. The results indicated that the porcine mammary gland secreted E(2). Increased concentrations of plasma E(2) collected only from P(4)-treated animals suggested that progesterone activated enzymes involved in steroidogenesis in porcine mammary gland, or those utilized in its metabolism.     

Effects of continuous elevated cortisol concentrations during oestrus on concentrations and patterns of progesterone, oestradiol and LH in the sow

This study investigated the effect of continuous elevated cortisol concentrations during standing oestrus on time of ovulation and patterns of progesterone, oestradiol and luteinising hormone (LH) in sows. The elevation of cortisol concentrations was achieved through repeated intravenous injections of synthetic adrenocorticotropic hormone (ACTH) every 2 h for approximately 48 h, from the onset of the second standing oestrus after weaning. Treatment was terminated when ovulation was detected (monitored by transrectal ultrasonography every 4h) or when the sow had received a maximum of 24 injections. The dose of ACTH (2.5 microg/kg) was chosen to mimic the cortisol concentrations seen during mixing of unfamiliar sows. The sows (n=14) were surgically fitted with jugular vein catheters and randomly divided into a control (C group where only NaCl solution were injected) or an ACTH group. Blood samples were collected every 2 h. In parallel with the blood sampling, saliva samples for cortisol analyses were taken from eight sows before onset of treatment and from four of the sows during treatment. There was no difference in time from onset of standing oestrus to ovulation between the two groups. The interval between the peaks of oestradiol and LH to ovulation was prolonged in the ACTH group compared to the C group (p<0.05), with a tendency towards an earlier decline of oestradiol in the ACTH group. Cortisol and progesterone concentrations were significantly elevated during treatment in the ACTH group (p<0.001), with cortisol peak concentrations occurring between 40 and 80 min after each ACTH injection. Cortisol concentrations in saliva and plasma were highly correlated (p<0.001). In conclusion, elevated cortisol concentrations from the onset of standing oestrus increase progesterone concentrations and prolong the interval between oestradiol and LH peaks to ovulation, the latter possible due to an early decline in oestradiol concentrations and a change of the LH peak outline. The effect these hormonal changes have on reproductive performance need to be further investigated. Saliva samples might be a useful and non-invasive method to assess cortisol concentrations in sows.     

Evidence for an oscillator other than luteinizing hormone controlling the secretion of progesterone in cattle

The objective of this study was to quantify and compare the frequencies of pulses in ovarian and systemic concentrations of progesterone, systemic concentrations of luteinizing hormone (LH) and rate of ovarian blood flow. Blood was collected simultaneously from previously implanted catheters in the ovarian venous circulation and jugular vein on Day 12 or 13 of estrous cycles from 4 nonlactating dairy cows. Blood was collected at a rate of 2.5 ml/min for 5 min out of every 10 min over an 8-h period. The mean rate of blood flow in the ovarian artery during the 5-min collection period was estimated by an electromagnetic blood flow transducer. Pulses were observed over time in both ovarian and systemic concentrations of progesterone at frequencies that ranged between 0.625 and 0.875 cycles/h (1.1 to 1.5 h/cycle) among the animals. Only one or two episodes of release of LH were observed during the 8-h period, and transient increases in blood flow to the ovaries were associated temporally with each episode of LH release. The estimated frequencies for release of LH and increased blood flow were the same for each animal and ranged between 0.250 and 0.375 cycles/h. A second cycle with a frequency similar to that for LH was evident in the spectral density functions for ovarian and systemic concentrations of progesterone. This cycle was eliminated when the cycle for LH was removed from the data for progesterone, but the magnitude and frequency of the pulses in progesterone were not affected.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pattern of ovarian progesterone secretion during the luteal phase of the ovine estrous cycle

Pulsatile secretion of progesterone has been observed during the late luteal phase of the menstrual cycle in the rhesus monkey and human. As the luteal phase progresses in each of these species, there is a pattern of decreased frequency and increased amplitude of progesterone pulses. The present study was designed to determine the pattern of progesterone secretion during the late luteal phase (Days 10-16) of the normal ovine estrous cycle. Five unanesthetized ewes, each bearing an indwelling cannula in the utero-ovarian vein, were bled every 15 min from 0800 h on Day 10 through 0800 h on Day 16 of the estrous cycle. With the computer program PULSAR, it was determined that progesterone secretion was episodic, with pulsations observed on all days. Analysis of variance was used to determine differences in frequency, amplitude, and interpeak interval (IPI) of progesterone pulses among ewes and days. The ewes averaged 8.0 +/- 0.63 pulses of progesterone per 24 h. Mean frequency of pulses was not different between days but showed differences between ewes. Mean amplitude of progesterone pulses was 7.0 +/- 0.27 ng/ml, with no differences observed either between days or between ewes. Mean IPI was 197 +/- 7.1 min, and, like frequency, the IPI was not different between days, but varied between ewes. No consistent temporal relationship was found between progesterone pulses and luteinizing hormone (LH), as determined by bioassay and radioimmunoassay, on Day 14 of the cycle in one ewe. The results indicate that progesterone secretion is episodic during the luteal phase of the ovine estrous cycle and is independent of LH pulses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sex hormones correlated with sex skin swelling and rectal temperature during the menstrual cycle of the pigtail macaque (Macaca nemestrina).

Daily measurement of serum luteinizing hormone, estradiol-17beta, and progesterone were made during the menstrual cycle in nine pigtail macaques (Macaca nemestrina). All data were normalized to the day of the luteinizing hormone peak. Serum estradiol-17beta increased from approximately 100 pg/ml during the early follicular phase to 442 +/- 156 pg/ml during the maximum midcycle concomitant with the luteinizing hormone peak, and a small increase in serum estradiol-17beta was observed during the luteal phase coincident with the progesterone peak. Serum progesterone values increased slightly at the time of the luteinizing hormone peak and increased from 0.2-0.3 ng/ml during the midfollicular phase to peak levels of 8.3 +/- 1.75 ng/ml 9 days after the luteinizing hormone surge. Serum luteinizing hormone remained low and relatively constant throughout the early and midcycle, then sharply increased approximately four-fold to peak values of 6.25 +/- 0.9 ng/ml. Sex skin swelling increased slowly during the follicular phase and declined slowly throughout the early luteal phase. Rectal temperature did not change significantly throughout the menstrual cycle. The similarity of plasma sex hormone changes during the menstrual cycle between women and the pigtail macaque suggested that this nonhuman primate should be a useful animal model for studying human reproduction.     

The biological meaning of progesterone levels

The effect of systematically delayed progesterone treatment was examined in 45 pregnant rats near term. Progesterone (P) and prostaglandin F (PGF) were measured in uterine vein plasma and uterine tissue before and during spontaneous labor or during prolonged pregnancy. Control animals exhibited the expected P-withdrawal (Pw) prior to spontaneous labor and properly time P-treatment predictably prevente Pw and labor. However, when P was administered 11.7 ± 2.8 hours (Mean ± S.E.) before spontaneous labor, the animals delivered normally despite increased plasma and tissue P-levels. These observations show that P-concentration can not be equated to P-action. Thus, when high P-levels are measured near term, as in parturient women, the biological ACTION of this hormone on uterine function should be cautiously interpreted.     

Plasma follicle stimulating hormone and luteinizing hormone patterns during the estrous cycle of ewes

Fifteen Suffolk ewes were used in three experiments to compare plasma follicle stimulating hormone (FSH) and luteinizing hormone (LH) patterns during the estrous cycle and to determine whether FSH levels undergo changes in pulse frequency. Luteinizing hormone changed inversely with progesterone levels whereas FSH and progesterone concentrations revealed no obvious relationship. Unlike LH, FSH levels did not pulsate during the follicular phase. Higher FSH levels were detected on days 1, 6 and 12 and lower levels on days 0, 4 and 16. Coincident preovulatory LH and FSH surges were observed and this was the only time FSH and LH levels appeared to be jointly controlled.     

8-hour rhythm of ovarian progesterone secretion in ewe

Pulse character of hormones secretion in the hypothalamus-pituitary-gonads system is a necessary condition of physiological regulation of reproduction. At the same time, the rhythms of ovarian hormones secretion have not been adequately explored. The researches study mainly three sexually mature ewes. The stages of oestrus cycle were determined on behavioral reactions of females in the presence of ram. Blood samples from jugular vein were collected hourly over 24-hour period during follicular (15-16 days), early (3-4 days) and middle (7-9 days) luteal phase of oestrus cycle, pregnancy (40-105 days) and lactation (30-45 days). 27 experiments were performed. Plasma progesterone was determined by enzyme-immunoassay method. There was no diurnal rhythm of ovarian progesterone secretion in ewes. During early and middle luteal phase of oestrus cycle and lactation, an 8-hour rhythm of progesterone secretion was detected. Follicular phase of oestrus cycle and pregnancy were characterized by irregular rises of fluctuations of progesterone level. It seems that the 8-hour rhythm of progesterone secretion during luteal phase and lactation is controlled by action of intraovarian generator of ultradian rhythms.     

Ovarian sympathectomy in the guinea pig

The influence of ovarian adrenergic nerves on follicular growth during the estrous cycle in the adult guinea pig was ascertained by comparing follicular development in control and chemically sympathectomized ovaries from the same animal. Selective ovarian sympathectomy was achieved by injecting 6-hydroxydopamine into a surgically closed periovarian membranous sac (bursa) on day 2 of the cycle (day 1 = day of estrus). The contralateral surgically closed ovarian bursa was injected with solvent used for 6-hydroxydopamine. Animals were laparotomized on days 5, 10 and 14 of the cycle. Blood from the utero-ovarian vein was collected bilaterally for measurement of progesterone and androstenedione. The ovaries were processed for histologic examination, and the number of follicles in each ovary was analyzed morphometrically. Sympathectomy on day 2 caused a decrease in healthy preovulatory follicles (greater than 700 micron diameter) on day 10 of the cycle. There were no differences in ovarian weights or the total number of follicles per ovary at this time. On days 5 and 14 of the cycle, there were no differences in ovarian weights, total number of follicles per ovary or follicles in any size classification. Sympathectomy did not alter progesterone levels in the utero- ovarian vein as compared to contralateral control levels. From control ovaries, there was a significant increase in progesterone in the blood of the utero-ovarian vein on day 10 but venous levels of progesterone from sympathectomized ovaries were not significantly different at any day of the cycle. In the venous effluent from sympathectomized ovaries, androstenedione was elevated at day 5 compared to days 10 and 14.(ABSTRACT TRUNCATED AT 250 WORDS)     

Levels of prostaglandin F (PGF) in bovine endometrium uterine venous, ovarian arterial and jugular plasma during the estrous cycle (38789).

Prostaglandins F (PGF) were measured in uterine vein, ovarian artery, and jugular vein plasma and in the endometrial tissues at various times during the bovine estrous cycle, and were compared to peripheral plasma progesterone levels. Four groups of heifers at days 1-5, 10-14, 15-17 and 20-0 of the estrous cycle were studied. Low levels of PGF (48 plus or minus 12 ng/g dry tissue were measured in the endometrium on days 1-14 of the cycle. Higher values (131 plus or minus 9.0) were found at days 15 until the day of estrus (p less than 0.001). Similarly, very low levels of PGF were observed in the uterine vein plasma at days 1-14 (0.162) plus or minus 0.044) ng/mlM plus or minus S.E.), whereas on days 15 until the day of estrus the levels ranged from 1.5 to 3.0 ng/ml. The increases in uterine vein PGF on day 15 occurred even while peripheral plasma progesterone levels were still high. However, PGF was not elevated in either the ovarian artery or the jugular vein at any time during the cycle, even when uterine vein levels were greatly elevated. No differences in PGF content were detected in endometrial tissue from uterine horns adjacent or opposite to the functional corpus luteum.     

High correlation between prolactinemia, 125-I hLH binding and progesterone secretion by an experimental luteoma

Autoimplantation of an ovary, containing fresh corpora lutea, into the spleen of an ovariectomized rat is followed by strong luteinization and size increase of the grafted gonad. Thus, large amounts of luteal tissue for biochemical studies, and their histological controls are available. Furthermore, progesterone secretion can be easily determined in samples collected from the portal vein. Since prolactin has been implicated in the control of luteal tissue, the role of this hormone on hLH binding and progesterone secretion was determined. Different levels of endogenous serum prolactin were achieved by pharmacological treatments with neurotropic agents. Scatchard plots of 125-I hLH binding data derived from luteoma particulate fractions revealed the presence of one type of binding site with high affinity. At the same time as binding increased, prolactinemia augmented, with a high correlation (R:0.99) between prolactinemia and LH binding. Moreover, progesterone secreted by the luteoma increased as LH binding sites augmented (R:0,97). It is concluded that a high correlation between prolactinemia and LH binding, as well as between this last parameter and progesterone output exists in the experimental luteoma.     

Effect of ACTH-challenge on progesterone and cortisol levels in ovariectomised repeat breeder heifers

In order to investigate the potential influence of stress as a component of the repeat breeding syndrome, the adrenocortical capacity for steroid production was evaluated in ovariectomised dairy heifers. In repeat breeder heifers (RBH), marginally elevated plasma progesterone levels during oestrus, so-called suprabasal progesterone levels, have earlier been measured and are believed to impair fertility. The aim was to distinguish if this progesterone could be of extra-gonadal or in this case, adrenal origin. Baseline levels of plasma cortisol and progesterone were determined as well as the corresponding response after induced acute stress in the form of an adrenocorticotropin (ACTH)-challenge. Comparisons were made between strictly selected RBH, n=5 and virgin heifers (VH), n=5 of the Swedish Red and White breed. The heifers were used as their own pre-challenge controls in a 2-day trial. On the control day, saline was injected i.v. and on the treatment day, a synthetic analogue of ACTH (60 microg Synachten(R)). Via a jugular vein catheter, blood samples were collected every 30 min for 6 h each day of the experiment. Analyses for plasma progesterone and cortisol were made. RBH had a significantly higher (P<0.01) pretreatment baseline cortisol level (10.1+/-2.3 nmol l(-1)) than VH (2.6+/-0.2 nmol l(-1)). Moreover, the cortisol response after stimuli was stronger in RBH than VH, especially concerning total hormone production (P<0. 001), but there was also a tendency towards higher peak values (P=0. 06) and longer duration of significantly increased hormone concentrations (P=0.08). Progesterone concentrations, however, did not differ between the groups. Both baseline levels (P=0.25) and posttreatment production (P=0.45) were of the same magnitude in RBH and VH. In conclusion, the study could not confirm that suprabasal progesterone concentrations during oestrus in RBH derive from the adrenal glands. Still, apparent differences were found in adrenocortical activity when ovariectomised heifers, VH and RBH, were subjected to an ACTH-challenge. It is suggested that a sustained adrenal stimulation associated with environmental or social stress could be one factor in the repeat breeding syndrome.     

Effects of arterial infusions of adrenalin and acetylcholine on luteal secretion of progesterone and oxytocin in goats

The effects of close intra-arterial infusion of acetylcholine and adrenalin on ovarian secretion of progesterone and oxytocin were examined on Day 10 of the estrous cycle in goats (estrus = Day 0). Acetylcholine (15 micrograms/min) was without effect, but adrenalin (10 micrograms/min) significantly (P < 0.001) raised both progesterone and oxytocin concentrations in ovarian vein plasma. These results show that luteal hormone secretion is enhanced in the goat by beta-adrenergic stimulation and suggest that, as in the sheep and cow, there may be neuroendocrine involvement in the regulation of caprine luteal function.     

Circadian profiles of progesterone,gonadotropins, cortisol and corticotropin in cycling and postmenopausal women

Little is known about the regulation of temporal variations of progesterone over the 24-hr span in young cycling women as well as in postmenopausal women. The purpose of the present study was to investigate the relationships between diurnal variations of progesterone and diurnal variations of hormones of the gonadotropic and corticotropic axes, and to provide further information on the source of progesterone secretion under physiological conditions. Twenty-four-hour hormonal profiles were explored under well-controlled laboratory conditions in 10 healthy women (21–36 yr old) with normal ovulatory cycles during early-mid follicular and late luteal phases, and in 8 healthy postmenopausal women (48–74 yr old). In young cycling women, significant positive relationships were found between progesterone and follicle-stimulating hormone (FSH) – but not luteinizing hormone (LH) – profiles during late luteal phase. Conversely, during follicular phase, significant positive relationships were evidenced between progesterone and cortisol profiles, but not between progesterone and FSH or LH. In postmenopausal women, strong positive correlations were found between progesterone and corticotropin (ACTH) or cortisol profiles. The present results indicate that during late luteal phase, temporal progesterone profiles are associated with FSH rather than with LH profiles. They also provide evidence that adrenal cortex is a major – or possibly the only – source of progesterone production during the follicular phase of the normal ovulatory cycle, and probably the only source after menopause.     

Progesterone does not affect the amount of mRNA for gonadotropins in the anterior pituitary gland of ovariectomized ewes

To evaluate the effect of progesterone on the synthesis and secretion of gonadotropins, ovariectomized ewes either were treated with progesterone (n = 5) for 3 wk or served as controls (n = 5) during the anestrous season. After treatment for 3 wk, blood samples were collected from progesterone-treated and ovariectomized ewes. After collection of blood samples, hypothalamic and hypophyseal tissues were collected from all ewes. Half of each pituitary was used to determine the content of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), and the number of receptors for gonadotropin-releasing hormone (GnRH). The amounts of mRNA for LH beta subunit, FSH beta subunit, alpha subunit, growth hormone, and prolactin were measured in the other half of each pituitary. Treatment with progesterone reduced mean serum concentrations of LH (p less than 0.001) but ot FSH (p greater than 0.05). Further, progesterone decreased (p less than 0.05) the total number of pulses of LH. We were unable to detect pulsatile release of FSH. Hypothalamic content of GnRH, number of receptors for GnRH, pituitary content of gonadotropins and mRNA for LH beta subunit, FSH beta subunit, alpha subunit, growth hormone, and prolactin were not affected (p greater than 0.05) by treatment with progesterone. Thus, after treatment with progesterone, serum concentrations of LH (but not FSH) are decreased. This effect, however, is not due to a decrease in the steady-state amount of mRNA for LH beta or alpha subunits.