Effects of progesterone-impregnated sponge treatment on peripheral plasma hormone levels and fertility in the cyclic ewe

Cyclic ewes were treated with 500-mg progesterone-impregnated sponges for the synchronization of oestrus. In the first experiment, the sponges were removed from 79 ewes at intervals over a 17-day treatment and the residual amount of progesterone was measured in order to assess the rate of absorption of the hormone from the sponges by the animals. The residual progesterone was found to decrease linearly with the duration of sponge insertion but there was also a significant quadratic component indicative of a slowing down in the rate of progesterone absorption towards the end of the treatment period. In the second experiment, 13 cyclic ewes were treated with 500-mg progesterone sponges for 17 days and the eight ewes in oestrus following spongewithdrawal were mated. The peripheral plasma progesterone was assayed at intervals during sponge insertion and at weekly intervals after sponge withdrawal. The residual progesterone levels on the sponges and the plasma progesterone levels of the treated ewes were examined in relation to their oestrous response and fertility. There was a significantly higher residual level of hormone remaining on sponges from ewes that mated than on sponges from those that did not (P < 0.01). The 13 ewes exhibited luteal phase levels of plasma progesterone when assayed during the period of sponge insertion regardless of their response to treatment. The mated, fertile ewes had significantly higher plasma hormone levels than the non-mated and the mated infertile ewes, after sponge withdrawal.     

Plasma progesterone levels during pregnancy in the little brown bat Myotis lucifugus (Vespertilionidae)

Plasma progesterone was measured by radioimmunoassay in individual female Myotis lucifugus throughout pregnancy and lactation. Progesterone levels, which averaged 6.7 +/- 0.7 ng/ml in late hibernation, rose to a mean of 18.9 +/- 6.7 ng/ml in unimplanted bats collected in the first two weeks after arrival at a maternity roost. Analysis of progesterone levels in bats in which the developmental stage of the embryo was known revealed two sharp, transient increases in plasma progesterone during the preimplantation period. The first, with values of 30-45 ng/ml, occurred at ovulation. The second, with values of 20-30 ng/ml, coincided with blastocyst formation. Progesterone levels increased exponentially from a mean of 7.4 +/- 1.0 ng/ml during early implantation to peak values of 100-200 ng/ml (means = 136.2 +/- 15.6) in the last two weeks of pregnancy, and showed no evidence of either a midpregnancy or prepartum decline. Despite involution of the corpus luteum at the end of pregnancy, progesterone levels averaged 9.0 +/- 1.0 ng/ml during lactation and did not decline until the end of lactation. In bats undergoing abortion, mean levels of plasma progesterone were already less than 6 ng/ml, equivalent to levels in nonbreeding females. The results indicate that the progesterone profile of pregnant M. lucifugus, though generally resembling those of other bats, exhibits several distinctive features. The sharp rise in plasma progesterone coinciding with blastocyst formation has not been reported in other mammals and suggests a possible role of progesterone in the cavitation process. In addition, peak values of plasma progesterone in late pregnancy were conspicuously higher than levels reported in other verpertilionid bats. The levels did not appear to fall before parturition, although such falls have been reported in other bats.     

Effects of progesterone on prolactin secretion in hypogonadal women

Although estrogen is known to stimulate the secretion of prolactin, there are only slight differences between the prolactin levels in the follicular and luteal phases in normal women. To test the hypothesis that progesterone is involved in the regulation of prolactin release, 50 mg of progesterone was administered intramuscularly at 0600 h to twelve hypogonadal women and blood samples were obtained at 15 min intervals between 1500 and 2000 h to determine the prolactin levels. The day before progesterone treatment, control blood samples were obtained at 15 min intervals between 1500 and 2000 h. The serum progesterone levels were 28.7 +/- 4.1 ng/ml at 1500 h, 24.2 +/- 3.5 ng/ml at 1730 h and 21.3 +/- 2.9 ng/ml (mean +/- SD) at 2000 h. In eight of twelve hypogonadal women, progesterone lowered circulating prolactin levels significantly. These results indicate that a high level of progesterone in the luteal phase may partly block estrogen-induced prolactin release physiologically.     

Effects of the antiandrogen hydroxyflutamide on progesterone secretion by preovulatory rat follicles in vivo and in vitro.

Serum and ovarian progesterone levels and in vitro production of progesterone by preovulatory follicles were measured on proestrus in pregnant mare's serum gonadotropin (PMSG) primed immature rats in which the luteinizing hormone (LH) surge and ovulation were blocked by administration of the antiandrogen hydroxyflutamide. Serum progesterone levels observed at 12:00 on proestrus were significantly elevated, twofold above those observed in vehicle-treated controls, by in vivo administration of 5 mg hydroxyflutamide 4 h earlier. In control rats, proestrous progesterone did not increase until 16:00, in parallel with rising LH levels of the LH surge. No LH surge occurred in the hydroxyflutamide-treated rats, ovulation was blocked, and serum progesterone declined throughout the afternoon of proestrus, from the elevated levels present at 12:00. Administration of human chorionic gonadotropin (hCG) at 11:00 advanced the elevation of serum progesterone by 2 h in vehicle-treated controls and prevented the decline in progesterone levels in hydroxyflutamide-treated rats. The patterns of change in ovarian tissue concentrations with time and treatment were essentially similar to those observed for serum progesterone. In in vitro experiments, progesterone secretion during 24 h culture of preovulatory follicles obtained on PMSG-induced proestrus was significantly increased, sixfold, by addition to the culture media of 370 microM but not of 37 microM hydroxyflutamide. Testosterone (50 nM) and hCG (20 mIU/mL) caused 26- and 14-fold increases, respectively, in progesterone secretion by cultured follicles. Hydroxyflutamide significantly reduced the stimulatory effect of testosterone but not of hCG on progesterone secretion in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)     

Peripheral blood steroid concentrations in the preovulatory rabbit.

Levels of 4 steroids in peripheral serum samples obtained from groups of mature, female New Zealand rabbits were determined and these steroids concentrations were related to the time after coital stimulation. Only females showing vaginal spermatozoa were included. Blood samples were collected directly from the heart from 13 rabbits sac rificed immediately after mating (Oh) and from 8 at 2 hours' postcoitus, 10 at 6 hours' postcoitus, and 8 at 12 hours' postcoitus. Serum concentrations of progesterone, 17alpha-hydroxyprogesterone, testosterone, and 17beta-estradiol were determined by radioimmunoassay. Blood levels of all these steroids except progesterone rose to approxima tely twice Oh concentrations by 2 hours' postcoitus, then declined to about Oh levels by 6 and 12 hours after mating. Blood progesterone levels showed a 20-fold rise over Oh concentrations 2 hours' postcoitus and remained elevated at 6 hours and then declined to Oh levels by 12 hours. Previously published patterns of postcoital determinations are similar. Postcoital levels of 17alpha-hydroxyprogesterone in the rabbit had not been published previously.     

Rescue of the corpus luteum in human pregnancy

Rescue of the corpus luteum from its programmed senescence maintains progesterone production required for pregnancy. In primates, chorionic gonadotropin produced by the developing conceptus acts as the primary luteotrophic signal. The purpose of this research was to assess corpus luteum rescue by examining changes in daily urinary progesterone metabolite levels during the first week after implantation. We determined the variability in progesterone metabolite profiles and evaluated its relationship to early pregnancy loss in 120 naturally conceived human pregnancies, including 43 early pregnancy losses. In other primates, an abrupt increase in the progesterone metabolite occurs at the time of implantation. This pattern occurred in an estimated 45% of the pregnancies in the present study. In the remaining pregnancies, there was a delayed rise (18%), neither a rise or decline (22%), or a decline (15%) during the week after implantation. The estimated rate of early pregnancy loss increased across these categories (from 5% loss with an abrupt rise at implantation to 100% loss with progesterone metabolite decline). Low urinary hCG levels in early pregnancy were significant determinants of a decline in postimplantation progesterone metabolite. However, preimplantation steroid metabolite levels were not significant, suggesting no inherent problem with the corpus luteum. Examination of individual progesterone metabolite profiles in relation to hCG profiles also indicated that few losses were caused by corpus luteum failure. Delineating the functional importance of an abrupt progesterone rise at the time of implantation may provide new strategies for promoting successful implantation in assisted reproduction.     

Peripheral plasma levels of oestradiol-17 beta and progesterone in the bitch during the oestrous cycle, in normal pregnancy and after dexamethasone treatment.

Plasma oestradiol-17 beta concentrations in Labradors increased during pro-oestrus to an average maximal concentration of of 79-7 +/- 10-9 (S.D.) pg/ml, and then fell rapidly. In 6/7 bitches the peak occurred within 1 day of oestrus. No consistent changes in plasma oestradiol levels were observed during pregnancy and at parturition and the values were similar to those in late anoestrus. Plasma progesterone levels did not increase markedly during pro-oestrus. At oestrus, progesterone values rose and maximal concentrations, which varied from about 20 to about 55 ng/ml, were reached within a few days of the oestradiol peak. Plasma progesterone decreased in late pregnancy and in one of the three bitches studied in detail low or undetectable levels were reached 10 days before parturition. In the other two bitches an abrupt decrease in progesterone occurred just before parturition. Dexamethasone treatment (2 X 5 mg daily for 10 days) from Day 30 of pregnancy resulted in intrauterine death and resorption of the fetuses in the two bitches studied. Treatment from about Day 45 resulted in the birth of dead fetuses at Days 55 and 59 of pregnancy. The changes in plasma oestradiol levels were very small. No changes in plasma progesterone levels were seen when dexamethasone was given in late pregnancy, but an accelerated decline occurred after treatment in mid-pregnancy.     

Variability in the response of the rabbit uterus to progesterone as influenced by prolactin

Ovariectomized rabbits from different breeders were treated at different times of the year with prolactin alone or with progesterone and the production of uteroglobin by the uterus was studied. There were seasonal, strain and dose variables in the uterine response to prolactin and progesterone. Treatment with prolactin (at 1 mg/day) plus progesterone generally induced higher levels of uteroglobin production than did treatment with progesterone alone. The differences were greatest in the winter for Tennessee animals and in the spring for animals from the New Mexico and North Carolina colonies. Ovariectomy produced a decrease (P less than 0.01) in the concentration of cytosolic oestrogen and progesterone receptors, and prolactin treatment restored the concentration to oestrous control values. However, there were no seasonally dependent changes in the concentration of the receptors for any of the treatment groups. Increased doses of prolactin (2 mg/day) induced high levels of uteroglobin production and new proteins to appear in uterine secretions of long-term ovariectomized rabbits but much lower levels (10-11%) when given to pregnant does. Additional ovulations were also noted plus adverse effects on the embryos.     

Milk fat progesterone concentrations in goats and early pregnancy diagnosis

Blood and milk samples from foremilk during afternoon milking, were simultaneously collected from 285 dairy goats. In experiment 1, fiva cyclic goats were sampled daily for 21 days. In experiment 2, 280 females from 9 flocks were submitted to sampling 21 days after insemination. In addition, some milk samples were divided in two parts, after which one was frozen and the other kept at +4 degrees C until assay. Progesterone concentrations were measured in blood, whole milk and milk fat by radioimmunoassay. No difference in whole milk or fat progesterone levels was found between frozen and refrigerated milk samples. Milk butterfat progesterone concentrations paralleled those in plasma or whole milk throughout the estrous cycle and ranged from about 20 ng/ml at estrus to about 400 ng/ml in mid-luteal phase. The ratio of mid-luteal phase progesterone levels to those seen in the estrous period was over 20 in fat and in blood. This ratio was very much lower in whole milk. Consequently the determination of pregnant and non-pregnant goats from the samples collected 21 days after service was very much easier and accuracy was better when the progesterone content was assayed from milk fat than from whole milk. It was concluded that early pregnancy diagnosis in goats can be done routinely by determination of progesterone levels in milk fat.     

A method to evaluate various parameters of progesterone production in beef cows,using the levels of progesterone in peripheral blood sampled on specific days of the oestrous cycle

Progesterone concentrations were determined in blood samples taken sequentially each day during 85 oestrous cycles in six beef cows. The sum of progesterone levels in sequential daily samples for a complete oestrous cycle varied considerably between cycles within cows (range 30–150 ng), and the means of cycles differed markedly between cows (range 36–60 ng). There was a typical pattern of daily progesterone levels throughout the oestrous cycle, and marked fluctuations were evident when the sum of daily progesterone levels for each cycle were plotted sequentially against time. Significant correlations and linear regression equations are described for the relationship between the sum of daily progesterone levels for a complete cycle and the mean concentration of progesterone in various combinations of daily samples. Furthermore, significant correlations were also obtained between the sum of daily progesterone levels and the maximum daily progesterone level obtained (r = 0.87; P < 0.001), the number of daily progesterone values greater than 2 ng/ml of blood (r = 0.89; P < 0.001) and the mean of daily progesterone levels for days 7–15 of the cycle (r = 0.91; P < 0.001). An ability to gain an estimate of these parameters of progesterone production from a small number of blood samples collected at strategic times during the cycle would facilitate the examination of the relationships between the environment, progesterone level, and the subsequent fertility in a greater number of animals, and so provide for a more precise measure of the effects of experimental treatments.     

Oestrogen and progesterone concentrations in plasma and oviductal tissue of ewes exhibiting a natural or induced oestrus

Synchronisation of oestrus in Karagouniki ewes by administration of the standard dose of progesterone results in lower fertility than observed when these ewes ovulate naturally. This suggests that the optimum dose of progesterone may be breed dependent. The exogenous progesterone may perturb the concentrations of oestradiol-17beta and progesterone in blood plasma and the oviductal wall. This possibility was investigated using Karagouniki ewes allocated at random to three treatments (n=4 per treatment). Ewes were allowed to exhibit natural oestrus (N) or oestrus was synchronised by administration of 250 mg (LP) or 375 mg (HP) progesterone (subcutaneous implants) followed by PMSG at 8 mg/kg live weight i.m. 14 days later. Oestrus was observed using teaser rams. Blood samples were collected for plasma oestradiol-17beta and progesterone assay from the onset to the end of oestrus at 2 h intervals. The uterus of each ewe was recovered at the end of oestrus and samples of the oviductal wall were taken from both oviducts and prepared, separately, for progesterone and oestradiol-17beta assay. Statistical analysis was performed using univariate analysis of variance. Plasma oestradiol-17beta concentrations from the onset to the end of oestrus were highest for N ewes and lowest for HP ewes with the values for LP ewes occupying an intermediate position. The differences were significant (P<0.05) between HP and the other two treatments from 4 to 12 h after the onset of oestrus and then between all treatments until the end of oestrus. Plasma progesterone levels were similar and fairly constant from the onset to the end of oestrus for N and LP. The plasma progesterone levels for HP were significantly (P<0.05) higher than for the other two treatments throughout oestrus. In oviductal wall samples, the oestradiol-17beta concentration was significantly (P<0.05) higher for N ewes than for synchronised ewes and the levels were similar for LP and HP ewes. The concentration of oestradiol-17beta differed (P<0.05) between right and left oviducts for N ewes but not for ewes of either of the synchronised oestrus treatments. Progesterone concentrations in oviductal wall samples were highest (P<0.05) for HP ewes and the values for N and LP ewes were similar. The concentration of progesterone did not differ between right and left oviductal wall samples within treatments. It was concluded that the higher dose of exogenous progesterone perturbed the levels of oestradiol-17beta and progesterone in blood plasma and the oviductal wall, and this could explain the lower levels of fertility (relative to naturally occurring oestrus) observed when this protocol is used for Karagouniki ewes in practice.     

Progesterone Receptor Expression Declines in the Guinea Pig Uterus during Functional Progesterone Withdrawal and in Response to Prostaglandins

Progesterone withdrawal is essential for parturition, but the mechanism of this pivotal hormonal change is unclear in women and other mammals that give birth without a pre-labor drop in maternal progesterone levels. One possibility suggested by uterine tissue analyses and cell culture models is that progesterone receptor levels change at term decreasing the progesterone responsiveness of the myometrium, which causes progesterone withdrawal at the functional level and results in estrogen dominance enhancing uterine contractility. In this investigation we have explored whether receptor mediated functional progesterone withdrawal occurs during late pregnancy and labor in vivo. We have also determined whether prostaglandins that induce labor cause functional progesterone withdrawal by altering myometrial progesterone receptor expression. Pregnant guinea pigs were used, since this animal loses progesterone responsiveness at term and gives birth in the presence of high maternal progesterone level similarly to primates. We found that progesterone receptor mRNA and protein A and B expression decreased in the guinea pig uterus during the last third of gestation and in labor. Prostaglandin administration reduced while prostaglandin synthesis inhibitor treatment increased progesterone receptor A protein abundance. Estrogen receptor-1 protein levels remained unchanged during late gestation, in labor and after prostaglandin or prostaglandin synthesis inhibitor administration. Steroid receptor levels were higher in the non-pregnant than in the pregnant uterine horns. We conclude that the decreasing expression of both progesterone receptors A and B is a physiological mechanism of functional progesterone withdrawal in the guinea pig during late pregnancy and in labor. Further, prostaglandins administered exogenously or produced endogenously stimulate labor in part by suppressing uterine progesterone receptor A expression, which may cause functional progesterone withdrawal, promote estrogen dominance and foster myometrial contractions.     

Preovulatory secretion of progesterone, luteinizing hormone, and prolactin in 4-day and 5-day cycling rats

Timing of ovulation and changes in plasma progesterone, luteinizing hormone (LH), and prolactin (PRL) during periovulatory stages were determined in Holtzman rats exhibiting regular 4- or 5-day cycles under a daily artificial illumination from 0500 to 1900 h. The 5-day cycling rats ovulated between 0130 and 0930 h on estrus, whereas some of the 4-day cycling animals ovulated as early as about 0130 h and others as late as 1130 h on estrus. Onset time of preovulatory LH and progesterone surges was about 1500 h on proestrus in both the 4- and the 5-day cycling rats. Peak levels of plasma LH and progesterone were measured at 1700 to 1900 h on proestrus, while the first rises and peak values of plasma PRL were evident a few hours earlier than those of plasma LH in the rats with two cycle lengths. Plasma LH levels at 1900 h on proestrus as well as plasma progesterone levels at 1600 and 2300 h on proestrus and at 0130 and 0330 h on estrus were significantly lower in the 5-day cycling rats than in the 4-day cycling animals (p less than 0.05). In contrast, PRL levels from 1500 through 2300 h on proestrus remained consistently higher in 5-day cycling rats than in 4-day cycling rats, and significant differences in PRL levels between these rats were apparent at 1500, 1600, and 2100 h (p less than 0.05-0.01). Thus, these results demonstrate that the 5-day cycling rats exhibit the attenuated magnitude of LH surge accompanied by the augmented preovulatory PRL release, and that plasma progesterone levels reflect the magnitude of LH surge. A tentative working hypothesis concerning the etiology of the 5-day cycle has been proposed.     

Differential inhibition of regional gastrointestinal tissue to progesterone in the rat

The influence of progesterone on contractile activity of three gastrointestinal regional tissues was evaluated. Up to six dose levels of progesterone were administered subcutaneously to male rats daily for four days. Progesterone blood levels measured with radioimmunoassay on the fourth day revealed that the range of progesterone exposure to the male animals did not exceed the progesterone blood level peak reported during the normal hormonal cycle of female rats. Log-dose response curves indicate that esophageal, antral, and colonic tissues from progesterone treated animals showed a significant reduction in contractile activity compared to the corresponding tissue from non-progesterone treated control animals. Esophageal and colonic tissues were two and 12-fold, respectively, more sensitive to the inhibitory progesterone influence compared to antral tissue. This study supports the concept that normal circulating levels of progesterone may have an influence on specific gastrointestinal regional function in addition to the effects of progesterone blood levels during pregnancy.     

Progesterone and estradiol-17beta in the peripheral plasma of Brahman cows with single and twin pregnancies

Peripheral plasma samples from Brahman cows with single and twin pregnancies were assayed for progesterone and estradiol-17beta throughout pregnancy. The twin pregnancies were obtained by transfer of Friesian embryos to inseminated single-ovulating Brahman cows. The twin-bearing cows had significantly higher levels of progesterone at 8 and 36 weeks of pregnancy. There were no differences in estradiol-17beta levels until the pre-parturient rise which occurred earlier in twin pregnancies. Intra-muscular injection of progesterone had no measurable effect on peripheral plasma levels of estradiol-17beta in Charolais cows 31 - 32 weeks pregnant.     

Observations on progesterone production and clearance in normal pregnant and fetectomized rhesus monkeys (Macaca mulatta).

The metabolic clearance rate (MCR) and production rate (PR) of progesterone were measured in rhesus monkeys both before and during intravenous infusion of progesterone at rates which approximately doubled or tripled the peripheral plasma levels. The monkeys were normally pregnant or fetectomized and were studied during the second half of pregnancy. Raising the peripheral plasma levels did not significantly after the MCR or the PR of progesterone. We conclude that peripheral progesterone levels are not the factor which controls the PR of progesterone in rhesus monkeys.     

Peripheral plasma progesterone: a marker for determining cyclicity in yaks (Poephagus grunniens L.)

An attempt was made to determine cyclicity in yaks using plasma progesterone during the breeding and non-breeding seasons. Fifteen non-lactating yaks were used in this experiment. During the breeding season (July to November), blood samples were collected from 8 yaks at least twice weekly until estrus was observed and then at 2 days interval for 30 days. During the non-breeding season (February to March), blood samples were collected from the same number of yaks at 2-day interval for 30 days. Progesterone was determined in plasma samples by radioimmunoassay. During the breeding season, plasma progesterone at estrus was basal (< or = 0.2 ng/ml). Concentrations increased thereafter with a sharp increase during the late luteal phase, typically reaching peak levels around day 15. Concentrations then declined rapidly over the following 4 days, reaching basal levels at estrus. A high proportion (66.7%) of potential estrous periods (based on progesterone concentrations) went undetected, indicating that silent or weak estrus was a prominent problem in yak cows. During the non-breeding season, three animals were found to be cycling as determined by the patterns of plasma progesterone. Yet, behavioral symptoms of estrus were not observed in any of these yak cows. We conclude that peripheral plasma progesterone concentrations can be used to monitor cyclicity in yak cows effectively.     

Effects of vanadyl sulphate on ornithine decarboxylase and progesterone levels in the ovary of rat

The effects of vanadyl sulphate in vitro on the levels of ODC activity and progesterone synthesis in ovaries were studied. The levels of ODC in the ovaries were stimulated with high concentration of vanadyl sulphate and at low concentrations there was no change in the levels of ODC activity. On the contrary progesterone levels were stimulated with low concentrations of vanadyl sulphate and were inhibited at higher concentrations. Vanadyl sulphate showed additional stimulation of ODC activity, when it was added with hCG and caused inhibition of hCG induced progesterone biosynthesis. These results show that the effects of vanadyl sulphate on ODC and progesterone are different.     

The influence of some sample handling factors on progesterone and testosterone analysis in goats

This study validated the use of commercially available radioimmunoassay kits for measuring the circulating progesterone and testosterone levels of goats. Progesterone and testosterone levels were then assayed in plasma which was collected from 23 does and 8 bucks. Collections from each animal were divided into three sodium fluoride-potassium oxalate (F/OX), one heparin, and one EDTA tubes and also into a tube without anticoagulant. Plasma from an F/OX tube was separated immediately from the blood cells by centrifugation. Serum or plasma was also separated after storage for 24 hours with F/OX, heparin or EDTA anticoagulant at 22 degrees C or with F/OX at 5 degrees C. A significant decline in assayable progesterone occurred in samples stored at 22 degrees C with each anticoagulant used and in the serum sample. Samples stored at 5 degrees C for 24 hours with F/OX anticoagulant contained concentrations of progesterone which did not differ significantly from those in samples where plasma was removed immediately. Assayable testosterone did not change with the anticoagulant used or vary with the storage temperature when F/OX tubes were stored at 5 degrees C and 22 degrees C for 24 hours. Results indicate that sample storage does influence levels of measured progesterone but not testosterone in goats. Progesterone assay is best done on plasma which is immediately separated from blood cells or on samples which are stored at 5 degrees C.     

Hormonal control of implantation in guinea pigs

In the guinea pig, for which implantation is supposedly progesterone-dependent, actual hormonal requirements were assessed by measuring the levels of circulating estradiol and progesterone and correlating them with their content in the ovaries and uterus, and uterine concentrations of their receptors prior to, during, and immediately after implantation. Ovarian and uterine content and plasma levels of estradiol and progesterone, as well as uterine cytosolic receptors of these two hormones, were high at proestrus. Up to day 3 of pregnancy, estradiol remained high in peripheral plasma, ovarian and uterine tissues, but reached low levels at the time of implantation. The levels of progesterone showed a gradual increase in plasma and ovaries till the time of implantation, with the embryonic site of the uterus accumulating more of progesterone compared to estradiol. As pregnancy progressed, a gradual translocation of cytosolic to nuclear receptors occurred, both with estradiol and progesterone receptors. Comparing the receptor values for estradiol at each uterine site showed no significant alterations between embryonic and interembryonic cytosolic receptors. While significantly high levels of nuclear estradiol receptor were found at the inter-embryonic site on day 9 of pregnancy, the cytosolic and nuclear progesterone receptor concentrations were greater at the embryonic site on the same day. These findings demonstrated that the uterus is adequately exposed to estradiol and progesterone prior to ovulation and again in early pregnancy (day 1-3), thus facilitating implantation in the guinea pig (on days 7-8).