Persistent vaginal cornification in mice treated with estrogen prenatally.

Twelve of 14 female mice of the ICR strain which had received a single injection of 50 mug estradiol-17beta on day 17 of fetal life exhibited irreversible cornification or stratification of the vaginal epithelium which persisted after ovariectomy until sacrifice performed 42-48 days later. Eight of the 12 mice had corpora lutea in their ovaries removed at 3-5 months of age. A similar injection of estradiol on day 15 of fetal life induced irreversible cornification or stratification of the vaginal epithelium in 6 of 12 females and only one of the 6 had corpora lutea in its ovaries when removed at 3-5 months. Mice given the same dose of estradiol on the day of birth or at 3 days of postnatal age invariably had ovaries bearing follicles of varying sizes and hypertrophied interstitial tissue but no corpora lutea. Changes in the vaginal epithelium in these animals were less remarkable as compared to that in prenatally treated mice.     

Structural and functional changes in ovaries from adult mice treated with diethylstilboestrol in the neonatal period

Ovaries from 8-week-old female NMRI mice in different stages of the oestrous cycle, or from females neonatally treated with the synthetic oestrogen diethylstilboestrol (DES; 5-10(-6) micrograms daily for 5 days), were studied histologically and for the ability to synthesize steroids from [3H]pregnenolone in vitro. Daily doses of 10(-4) micrograms DES or higher resulted in absence of corpora lutea. In ovaries lacking corpora lutea, the interstitial tissue dominated and the cells in this compartment were large with a clear cytoplasm. The steroids synthesized in ovarian homogenates were separated with thin-layer chromatography. The homogeneity of the steroids was checked in recrystallization experiments. Daily doses of 5-10(-4) micrograms DES in the neonatal period resulted in pronounced deviations in the pattern of ovarian steroids synthesized as compared with control ovaries. In DES-exposed ovaries, the synthesis of androstenedione and, above all, progesterone was high while the synthesis of 17 alpha-hydroxyprogesterone and testosterone was reduced compared with controls. These results could argue for a difference in activities of 17 alpha-hydroxylase and 17 beta-ol-dehydrogenase in ovaries from DES-treated females compared with controls. After transplantation of DES-exposed ovaries to ovariectomized control females, the steroid pattern changed to that typical for control ovaries. Control ovaries transplanted to DES-treated females had a steroid pattern similar to that of DES-exposed ovaries.     

Polyovular follicles in mouse ovaries exposed neonatally to diethylstilbestrol in vivo and in vitro

In 35-day-old C57BL/Tw female mice given daily injections of 1 microgram diethylstilbestrol (DES) for 5 days from the day of birth, a significantly higher incidence of polyovular follicles (PF) were found in the ovaries than in those of age-matched control mice. Ovaries of prepubertal mice treated neonatally with oil or DES (DES mice) showed an enhancement of ovulation and luteinization following a combined treatment with eCG and hCG. Tubal ova in DES mice treated with eCG plus hCG were surrounded by many granulosa cells. Incidence of PF in control mice was not changed by eCG plus hCG treatment. In contrast, PF incidence in DES mice was reduced by prepubertal injections of eCG plus hCG. A high incidence of PF was also found in newborn mouse ovaries transplanted for 30 days into ovariectomized adult hosts given DES injections, but not in ovaries transplanted into intact or ovariectomized DES-untreated hosts. When neonatal ovaries were cultivated in a serum-free medium containing DES for 5 days and then transplanted into ovariectomized hosts, PF were formed in the grafts, but not in DES-unexposed grafts. Oocytes from PF in DES mice were found to have a smaller capacity for fertilization when examined in vitro. The present study also demonstrated that neonatal ovaries exposed to estrogen in vivo or in vitro (which produces PF in prepubertal hosts) are capable of responding to gonadotropins given later, resulting in a reduction of PF incidence, and that exogenous estrogen acts directly on neonatal ovaries to induce PF.     

The ovary of the wood mouse, Apodemus sylvaticus, during late pregnancy and lactation

Female wood mice, Apodemus sylvaticus, were killed on day 18 of pregnancy (P 18) and on days 0, 3, 6, 9, 12, 15, and 18 of lactation (L 0, L 3, L 6, L 9, L 12, L 15, and L 18 respectively), and the ovaries were studied. The weight of the ovaries was recorded at dissection. The corpora lutea and the follicles of the right ovary were counted and measured, and the appearance of the interstitial tissue was noted. A decline in weight from day P 18 to day L 6 coincided with a decrease in mean diameter of the corpora lutea. The mean number of corpora lutea did, however, not change over the period. The corpora lutea present throughout lactation were probably from the gestation period; the females did not appear to ovulate post-partum. The interstitial tissue was not affected, as far as could be judged with light microscopy. Ovulatory follicles were only present at times close to expected ovulation; on days P 18 and L 18. A lactational anoestrous is suggested for the wood mouse.     

Reappearance of cyclicity of vaginal smears in androgen-sterilized rats following thyroidectomy

During studies on the mechanism of hypersensitivity to gonadotropins of thyroidectomized rat ovary, results were obtained which suggest an increase in the endogenous luteinizing hormone (LH) release after thyroidectomy in androgen-sterilized rats. Female rats were injected subcutaneously with 1 mg of testosterone propionate dissolved in .05 ml of olive oil on Day 5 after birth. At the age of 10-12 weeks, those animals which showed persistent vaginal cornification were thyroidectomized. Within 1-2 days after thyroidectomy, the thyroidectomized rats exhibited leukocytic and epithelial vaginal smears for 2-6 days. Irregular cyclicity with the pattern of 2-6 days diestrus and 3-10 days estrus persisted for 1 month. Histological examination revealed that the corpora lutea were intermingled with a number of cystic follicles in the ovaries of the androgen-sterilized and throidectomized rats while the ovaries of androgen-sterilized controls had vesicular follicles but were devoid of corpora lutea. The results indicate a rapid enhancement in the LH release in androgen-sterilized rats following thyroidectomy.     

Frequent occurrence of polyovular follicles in ovaries of mice exposed neonatally to diethylstilbestrol

The occurrence of polyovular follicles (PF) was examined at 10-34 days of age in the ovaries of BALB/cCrgl female mice given five daily injections of 0.1 microgram diethylstilbestrol (DES), 2 micrograms DES, 100 micrograms progesterone (P), 137 micrograms 17 alpha-hydroxyprogesterone caproate (HPC), 20 micrograms testosterone (T), 20 micrograms 5 alpha-dihydrotestosterone (5 alpha-DHT), or oil vehicle alone starting on the day of birth, and of C57BL/Tw females given five neonatal injections of 1 microgram DES, 20 micrograms 17 beta-estradiol (E2), 50 micrograms 5 alpha-DHT, 50 micrograms 5 beta-DHT, or the vehicle alone. Ovaries of 30-day-old C57BL mice given five daily injections of 1 microgram DES starting at 3-25 days of age were also examined. PF incidence (% of PF per ovary) and PF frequency (% of mice with PF) were significantly greater in BALB/c mice receiving injections of DES, P, HPC, and T than in the controls. In DES-treated mice at 34 days, PF incidence (2-13 oocytes/follicle) was 120-340 times higher than in the controls. BALB/c mice treated with T, P, and HPC showed PF incidence (two to four oocytes/follicle) three- to six-fold higher than in the controls. In 30-day-old C57BL mice treated with T, E2, and DES, PF incidence also increased by two- to 50-fold. 5 alpha-DHT and 5 beta-DHT failed to increase PF incidence. PF incidence was significantly increased only when neonatal DES treatment was begun on days 0 to 3, but was reduced when started at days 10-25.(ABSTRACT TRUNCATED AT 250 WORDS)     

Luteal and follicular count in bitches: assessment by means of magnetic resonance imaging

This study was done to determine whether preovulatory follicles or corpora lutea (physiological structures, PS) can be counted in the ovaries of bitches by means of magnetic resonance imaging (MRI). In Experiment 1, the ovaries from 15 German shepherd bitches (five in the follicular phase, one in the periovulatory period, five during the first 38 days of diestrus and four between Day 48 of diestrus and full-term gestation) were embedded in gelatin to form three phantoms with 10 ovaries each. Each phantom was exposed to MRI, using a 1mm slice thickness, a 1mm slice interval, a voxel size of 1mm cubic and a variety of pulse sequences, whereafter the ovaries were dissected and the numbers of follicles, corpora lutea and cysts counted. T2-weighted images were superior to T1-weighted images. Each of three operators counted the numbers of PS and cysts on T2-weighted images obtained in the coronal, transverse and sagital planes of each ovary, which, for the 30 ovaries, provided 270 operator by ovary by plane estimations and 90 operator by ovary estimations for each type of structure. Images of cysts were hyperintense, those of early corpora lutea and follicles similar and moderate and those of late corpora lutea hypo-intense and not clearly discernable from ovarian stroma. Estimations of PS were too low in 68%, correct in 12% and too high in 20% of estimations (n=270). Estimations of PS were correct in three operator by ovary combinations, out by 1 in 22 and out by more than 1 in 65. No operator estimated PS correctly in any bitch. In Experiment 2 MRI was done on three deeply sedated bitches in the periovulatory phase in an attempt to obtain images of the ovaries in order to count the follicles. The acquisition time of 5-7 min rendered images of poor quality from live bitches and none of their ovaries could be seen. MRI is not suitable for counting follicles or corpora lutea in the ovaries of bitches.     

Inhibition of pituitary-gonadal axis in mice by long-term administration of D-Trp-6-LHRH microcapsules

Female mice were injected, every 30 days for 5 months, with a long-acting formulation of microcapsules liberating 2.5 micrograms D-Trp-6-LHRH/day. The control group was injected with vehicle only. At 30 days after the last injection mice were killed, ovaries, uteri and adrenals were weighed and fixed in formalin for histological studies. LH and oestradiol concentrations were measured by RIA. In the D-Trp-6-LHRH-treated group, the weights of the ovaries and uterus (P less than 0.01 and P less than 0.05, respectively), and LH and oestradiol values (P less than 0.02 and P less than 0.01, respectively) were reduced compared to controls. Histologically, the ovaries contained a large number of degenerated, atretic follicles, and corpora lutea had almost completely disappeared. These results indicate, contrary to the prevailing opinion, that mice are sensitive to inhibitory effects of LHRH agonists and that a suppression of the pituitary-gonadal axis can be obtained with long-term administration of D-Trp-6-LHRH microcapsules.     

Response of prepubertal ewes primed with monensin or progesterone to administration of FSH

Prepubertal ewe lambs were treated with FSH after progesterone priming for 12 days (Group P), monensin supplementation for 14 days (Group M) or a standard diet (Group C). Serial blood samples were taken for LH and progesterone assay, and ovariectomy was performed on half of each group 38-52 h after start of treatment to assess ovarian function, follicular steroid production in vitro and the concentration of gonadotrophin binding sites in follicles. The remaining ewe lambs were ovariectomized 8 days after FSH treatment to determine whether functional corpora lutea were present. FSH treatment was followed by a preovulatory LH surge which occurred significantly later (P less than 0.05) and was better synchronized in ewes in Groups P and M than in those in Group C. At 13-15 h after the LH surge significantly more large follicles were present on ovaries from Group P and M ewes than in Group C. Follicles greater than 5 mm diameter from ewes in Groups P and M produced significantly less oestrogen and testosterone and more dihydrotestosterone, and had significantly more hCG binding sites, than did similar-sized follicles from Group C animals. Ovariectomy on Day 8 after the completion of FSH treatment showed that ewes in Groups P and M had significantly greater numbers of functional corpora lutea. These results indicate that, in prepubertal ewes, progesterone priming and monensin supplementation may delay the preovulatory LH surge, allowing follicles developing after FSH treatment more time to mature before ovulation. This may result in better luteinization of ruptured follicles in these ewes, with the formation of functional corpora lutea.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hormonal Control of Sexual Differentiation of the Hypothalamus in the Neonatal Female Rat

Groups of female rats were treated either on the day of birth or at 5 days of age with oil, testosterone propionate (TP), MER-25 (an oestrogen antagonist) or TP plus MER-25. Treatment with oil and MER-25 alone on day 1 or 5 did not prevent the development of cyclic repro ductive activity in any of the females. Administration of TP on the day of birth inhibited vaginal opening and prevented cyclic ovarian activity. Treatment with TP on day 5 significantly advanced the time of vaginal opening but prevented both vaginal and ovarian cyclicity. Treatment on the day of birth with a combination of MER-25 and TP did not significantly prevent the masculinizing action of TP. In most cases vaginal opening failed to occur; the ovaries were small and devoid of corpora lutea. Sexual receptivity was not significantly affected by treatment on the day of birth with oil, TP or MER-25 alone. When MER-25 was given before the TP, however, receptivity was significantly enhanced compared with oil treatment. At 5 days of age treatment with MER-25 plus TP prevented the masculinizing action of TP. Most of the animals had normal ovarian weights, and ovaries which contained both follicles and corpora lutea. The most effective treatment for preventing the action of TP was administration of MER-25 6 h before the injection of androgen. After treatment at 5 days of age, sexual receptivity was significantly reduced in all groups compared with the oil-treated animals. Administration of MER-25 plus TP did not enhance sexual receptivity above that found in the animals receiving TP alone. The results are discussed in terms of the possible role of oestrogen in controlling hypothalamic differentiation in the neonatal female rat.     

A role for the androgen receptor in follicular atresia of estrogen receptor beta knockout mouse ovary.

Estrogen receptor beta (ERbeta) is highly expressed, but ERalpha is not detectable in granulosa cells in the mouse ovary. In ERbeta knockout (BERKO) mice, there is abnormal follicular development and very reduced fertility. At 3 wk of age, no significant morphologic differences were discernable between wild type (WT) and BERKO mouse ovaries, but by 5 mo of age, atretic follicles were abundant in BERKO mice and there were very few healthy late antral follicles or corpora lutea. At 2 yr of age, unlike the ovaries of their WT littermates, BERKO mouse ovaries were devoid of healthy follicles but had numerous large, foamy lipid-filled stromal cells. The late antral and atretic follicles in BERKO mice were characterized by a high level of expression of the androgen receptor (AR) and IGF-1 receptor. These proteins were abundantly expressed in granulosa cells of preantral and early antral follicles in both genotypes, but their expression was extinguished in late antral follicles of WT mice. Healthy late antral follicles and corpora lutea were restored in BERKO ovaries after 15 days of treatment of mice with the antiandrogen flutamide. The results suggest that in the absence of ERbeta there was a loss of regulation of AR. Because androgens enhance recruitment of primordial follicles into the growth pool and cause atresia of late antral follicles, the inappropriately high level of AR probably is related to the follicular atresia and to the early exhaustion of follicles in BERKO mice.     

Counteraction of testosterone-induced suppression of the pituitary-ovarian axis in rats by flutamide.

Daily administration of 100 micrograms of testosterone to unilaterally ovariectomized rats for 18 days caused a significant decrease in compensatory ovarian hypertrophy. The number of corpora lutea was markedly reduced and many cystic follicles were noticeable. Administration of graded doses (0.5, 1 and 3 mg daily) of flutamide over the same period did not cause any significant change in the weight and histology of the ovary in untreated unilaterally ovariectomized animals. However, treatment with the same doses of flutamide prevented the changes observed in the reamining ovary of testosterone-treated animals.     

Reduction in corpora lutea number in obese melanocortin-4-receptor-deficient mice

Obese melanocortin-4-receptor-deficient (MC4R-/-) male mice are reported to have erectile dysfunction, while homozygous MC4R-/- female mice are apparently fertile. A recently established obese mouse strain, carrying an inactivating mutation in the MC4R gene, revealed difficulties in breeding for the homozygous female mice. This prompted us to determine the presence of follicles and corpora lutea (CL) in ovaries of MC4R-/- mice aged 3–6 months in comparison to wild type (MC4R+/+) littermates. Serial sections of formaldehyde-fixed ovaries of mice with vaginal signs of estrus and metestrus were assessed for the number of healthy and regressing follicles and CL. The number of CL, as an estimate for the ovulation rate, decreased to zero during aging in MC4R-/- mice. The number of small- (diameter 100–200 micrometer) and large-sized follicles namely antral follicles (diameter >200 micrometer) were slightly increased in MC4R-/- compared to MC4R+/+ mice. Greater differences were found in very large to cystic follicles, which were more numerous in MC4R-/- mice. The number of regressing antral follicles was higher in the MC4R-/- group compared to the MC4R+/+ group. This was associated with a wide range in the number of collapsed zonae pellucidae as the last remnants of regressed follicles. A conspicuous hypertrophy of the interstitial cells was noted in 6-month-old MC4R-/- mice. In conclusion, cystic follicles and the reduction in CL number point to a decreased ovulation rate in obese MC4R-/- mice.     

Cyclic AMP-dependent protein kinase in mitochondria and cytosol from different-sized follicles and corpora lutea of porcine ovaries

cAMP-dependent protein kinase was examined in mitochondria and cytosol prepared from different-sized antral follicles and corpora lutea of porcine ovaries. In all ovarian tissues examined except small follicles, protein kinase-specific activity was significantly higher in mitochondria than in cytosol, with the highest to lowest activities being found in medium (4-6 mm) follicles, large (7-12 mm) follicles, corpora lutea, and small (1-3 mm) follicles, respectively. Using the photoaffinity analogue [32P]8-N3cAMP, two major cAMP binding proteins with Mr = 47,000 (the apparent regulatory subunit of protein kinase Type I) and 54,000-56,000 (Type II) were found in all ovarian preparations. Type II was predominant in the cytosol of all ovarian samples, with the cytosolic Type I to Type II ratio increasing from approximately 0.05 in small and medium follicles top approximately 0.20 in large follicles and corpora lutea. In contrast, ovarian mitochondrial preparations contained relatively more Type I than did cytosol, with the mitochondrial Type I to Type II ratio increasing from approximately 0.50 in small and medium follicles to 0.88 in large follicles and 2.96 in corpora lutea. Also, mitochondrial [4-14C]cholesterol conversion and 3 beta-hydroxysteroid dehydrogenase/isomerase activities increased with follicle size and luteinization. These results suggest that Type I may play a role in the regulation of ovarian mitochondrial steroidogenesis.     

Effects of neonatal treatment with phytoestrogens, genistein and daidzein, on sex difference in female rat brain function: estrous cycle and lordosis

It is well known that neonatal exposure to estrogen induces masculinization or defeminization of the brain. In this study, the effects of neonatal treatment with two kinds of soybean isoflavone aglycone, genistein (GS) and daidzein (DZ), on the estrous cycle and lordosis behavior were investigated. Female rats were injected subcutaneously with 1 mg GS, 1 mg DZ, 100 microg estradiol (E2), or oil daily for 5 days from birth. As a result, vaginal opening was advanced in GS- or E2-treated females. A vaginal smear check indicated that oil- or DZ-treated females showed a constant 4- or 5-day estrous cycle, whereas GS- or E2-treated rats showed a persistent or prolonged estrus. Ovariectomy was performed in all females at 60 days of age. The ovaries in the GS- or E2-treated groups were smaller than those in the oil- and DZ-treated groups and contained no corpora lutea. In the DZ group, although corpora lutea were seen, ovaries were smaller than that of control females. Behavioral tests were carried out after implantation of E2-tubes. All of the oil- or DZ-treated females showed lordosis with a high lordosis quotient (LQ). On the other hand, as male rats, LQs were extremely low in the E2-treated group, when compared to the oil-treated group. In the GS-treated group, the mean LQ was lower than that in the oil-treated group, but higher than those in the E2-treated female or male groups. These results suggest that genistein acts as an estrogen in the sexual differentiation of the brain and causes defeminization of the brain in regulating lordosis and the estrous cycle in rats. In addition, neonatal daidzein also has some influence on ovarian function.     

Effects of gonadotrophin on the ovary of Apodemus flavicollis in winter anoestrus

Wild-caught female Apodemus flavicollis were given daily subcutaneous injections of PMSG + HCG on two consecutive days (2xPMSG + HCG), HCG on two days (2 X HCG), PMSG + HCG until their vagina became perforate (Perforate), and PMSG + HCG until they became perforate and were left undisturbed for another five days (Perforate + 5). Untreated females served as controls. The ovaries and uteri increased in weight as a result of the treatment. The number of healthy follicles increased in "perforate" females and decreased in "2 X HCG". Luteinization of the follicles and possibly of the interstitial tissue was seen in "2 X HCG". In this group, the mean diameter of the corpora lutea increased, and it decreased in "perforate" and "perforate + 5". The mean number of corpora lutea was unaffected by the treatment. The activity of 3 beta hydroxysteroid dehydrogenase (3 beta HSD) was strong in the corpora lutea of the untreated females and remained so throughout treatment. The interstitial tissue of the untreated females showed only weak 3 beta HSD activity, but the activity was strong in all the experimental groups. One female ovulated, but it is not clear if it was in response to treatment.     

Immunocytochemical localization of aromatase in immature rat ovaries treated with PMSG and hCG,and in pregnant rat ovaries

Summary Immunocytochemical localization of aromatase cytochrome P-450 was examined in immature rat ovaries treated with pregnant mare serum gonadotropin (PMSG) and human chorionic gonadotropin (hCG), and in pregnant rat ovaries. It is well known that PMSG and hCG treatments induce ovulation about 12 h after hCG injection.At 24 h after hCG injection, many antral follicles were recognized in immature rat ovaries and only the granulosa cells in the antral follicles were stained weakly with the anti-aromatase antibody. At 0 to 9 h after hCG injection, in addition to the antral follicles, some large Graafian follicles could be observed in the rat ovaries, and the granulosa cells of these follicles were positively stained for aromatase. Each follicle was surrounded by the basal lamina which shows lineally distinct positive reaction against anti-laminin antibody. At 12 h after hCG injection, some large Graafian follicles without oocyte were weakly positive to the anti-aromatase antisera, and the outline of their basal lamina stained with anti-laminin antibody became irregular in shape and fragmentous. At 15 to 18 h after hCG injection, the luteinized cysts could be seen, and the granulosa-lutein cells of these cysts were almost negative for aromatase. Fragmentous reaction to the anti-laminin antibody was observed around the luteinized cysts.In the ovaries of day 4 in pregnancy, only the granulosa cells of the large antral follicles were weakly stained, but corpora lutea negatively reacted to the anti-aromatase antibody. At 7 to 19 days in gestation, both the granulosa cells of antral follicles and pregnant luteal cells were positively stained against aromatase antisera. The luteal cells were increased in size during pregnancy. And weakly positive reaction was detected on day 7 of pregnancy, then the immunoreaction became stronger in the corpora lutea on day 15 and 19 of pregnancy.The localization of aromatase was immunocytochemically examined in immature rat ovaries treated with PMSG and hCG injection, and the reaction of the granulosa cells of the antral follicles against anti-aromatase antibody became strongly positive about 12 h before ovulation and the became very weak suddenly after ovulation. In rat-ovaries, the pregnant corpora lutea was positively stained for aromatase after day 7 of pregnancy.This study was supported by Grants from the Ministry of Education, Science and Culture, Japan, and from USPHS Research Grants HD04945, USA     

An assessment of mast-cell deficient mice (W/Wv) as a model system to study the role of histamine in implantation and deciduoma formation

The ovaries from mast cell-normal (+/+) and mast cell-deficient (W/Wv) mice were examined with light and electron microscopy. In addition the effect of ovariectomy and subsequent steroid treatment on total uterine histamine content, total mast cell numbers and surface and glandular epithelial cell heights was measured. The ovaries of +/+ mice were normal, displaying various stages of follicular growth and atresia and numerous corpora lutea; the ovaries of W/Wv mice lacked follicles and corpora lutea but contained numerous hyperplastic interstitial cells which contained numerous lipid droplets, vesiculated mitochondria and abundant endoplasmic reticulum suggestive of steroid synthesis. Steroid treatment of ovariectomized +/+ and W/Wv mice caused a significant increase in uterine wet weight and endometrial surface and glandular epithelial cell heights. In +/+ mice, steroid treatment caused a concomitant increase in total mast cells per uterine horn while mast cells were totally absent in W/Wv mice. The increase in uterine histamine in +/+ mice is consistent with the increase in mast cell numbers. Measurable amounts of uterine histamine, which increases slightly after steroid treatment, were demonstrated in W/Wv mice. Since the uteri of +/+ and W/Wv mice respond to steroids in a similar manner with the sole exception being histamine content and mast cell numbers, our results demonstrate the potential of using these animals to investigate the role(s) of uterine mast cells and non-mast cell uterine histamine in the process of implantation and the formation of a decidual cell response.     

Ovarian response to pregnant kare serum gonadotrophin and prostaglandin F(2) proportional, variant in Africander and Mashona cows

Oestrus was synchronised in ten Africander and eight Mashona mature dry cows by two injections of prostaglandin F(2) proportional, variant (PG) 11 days apart. Half the cows of each breed received an injection of 3000 i.u. pregnant mare serum gonadotrophin (PMSG) two days prior to the second PG injection. All cows were observed for the incidence of cestrus, and blood samples were taken at intervals for progesterone assay. Cows were slaughtered 11 days after the second PG injection and their reproductive tracts examined. Treatment with PMSG increased numbers both of corpora lutea and of follicles more than 10 mm in diameter. When numbers of corpora lutea and follicles were considered together, the response to treatment was significant in the Africanders (P<0,01) and markedly greater than that of Kashona cows. The concentration of progesterone in plasma on the day before slaughter was significantly correlated with the mass of corpora lutea (P<0,001), total mass of ovaries (P<0,001), but not with numbers of corpora lutea. It is suggested that generally Africander cows may secrete lower levels of follicle stimulating hormone and oestrogen than kashona cows during normal cyclic sexual activity.