Administration Of Mestranol Research Articles

Effect of chronic administration of mestranol, tamoxifen, and toremifene on hepatic ploidy in rats.

The nonsteroidal antiestrogen tamoxifen increases the incidence of rat liver cancer through a variety of mechanisms. To compare the effects of tamoxifen (TAM) and a structurally similar analog toremifene (TOR) on rat liver, we determined the ploidy distribution for hepatocytes isolated from rats treated for 18 months with these antiestrogens or the estrogenic compound mestranol (MS). Female Sprague-Dawley rats were subjected to a 70% partial hepatectomy and administered the solvent, trioctanoin, or diethylnitrosamine (10 mg DEN/kg). After a 2-week recovery from the surgery, the rats were administered a basal diet or one containing TAM (250 or 500 ppm), TOR (250, 500, or 750 ppm), or MS (0.2 ppm) for 18 months. Pathologic changes in the liver were examined in the 15-22 rats per treatment group at the 18-month time point. An increased incidence of hepatocellular carcinomas (HCC) was detected in the 500 ppm TAM group, but not with the other treatments that did not include DEN. Both TOR and TAM promoted formation of DEN-initiated HCCs. At sacrifice, four to five rats per group were perfused and the hepatocytes isolated and cultured. Karyotypic analysis was performed on colcemid-blocked cells after 2 days in culture. The hepatic ploidy distribution was characterized in Giemsa-stained metaphase spreads. These studies indicated that chronic treatment with TAM alone resulted in a shift from tetraploid to diploid, as was also observed for rats treated once with DEN. TOR and MS alone did not cause this change in hepatic ploidy at the doses examined. A shift toward an increased content of diploid hepatocytes occurred in all rats treated once with DEN followed by TAM, TOR, or MS. These results indicate that tamoxifen administration results in a shift toward growth of diploid hepatocytes, thus contributing to its carcinogenic action in the rat liver.

Effect of the separate and combined administration of mestranol and phenobarbital on the development of altered hepatic foci expressing placental form of glutathione S-transferase in the rat.

The stages in the carcinogenesis process include initiation, promotion and progression. Although many characteristics of tumor promotion and promoting agents have been reported, relatively few studies on the effects of combinations of promoting agents have been detailed. For study of the combined effects of phenobarbital (PB) and mestranol (MS) in multistage rat hepatocarcinogenesis, an initiation-promotion protocol was developed. Female rats were injected i.p. at 5 days of age with either diethylnitrosamine (DEN) (10 mg/kg) or the solvent tricaprilyn. At weaning, approximately 10 rats from both the DEN-initiated and the solvent control groups were provided basal diet alone, PB (10, 100 or 500 mg/kg diet), MS (0.02 or 0.2 mg/kg diet) or various combinations of both PB and MS in the basal diet. At 8 months of age, the rats were killed and the livers removed, sectioned and fixed in ice-cold acetone. Sections of 5 microgram thickness were stained for placental glutathione S-transferase (PGST) expression, and the volume fraction of liver occupied by altered hepatic foci was determined by stereology. In addition, incorporation of bromodeoxyuridine into nuclei of PGST-expressing (focal) and non-PGST-expressing (non-focal) hepatocytes was determined. Administration of the highest dose of PB resulted in a significant decrease in non-focal hepatocyte labeling index, with a 4-fold differential between the focal and non-focal hepatocyte labeling index. Administration of 0.2 mg/kg diet MS resulted in effective promotion. The non-focal labeling index was increased and the focal labeling index was further enhanced (3-fold) relative to the non-focal index by this dose of MS. Combination of the lower MS dose with PB resulted in at least an additive promoting effect; however, a lower volume fraction was noted for the combination of low MS dose plus the highest PB dose. Combination of the higher MS dose with PB resulted in an elevation of volume fraction only for the middle PB dose. These findings indicate that the potency of promotion by mixtures is modulated by the dose of each component as well as by pharmacodynamic and pharmacokinetic properties of each component of the mixture.

EFFECTS OF ESTROGEN ON MOUSE LIVER SINUSOIDAL CELLS

The effects of estrogen on liver sinusoidal cells were investigated immunohistochemically and morphometrically. Male AKR/n mice were orally administered 30mg/kg/day of mestranol suspended in sesame oil for 7 days. Some of the mice were splenectomized before mestranol administration. Relative liver weight significantly increased from day l to day 7 in mestranol-administered mice with or without spleen. Relative spleen weight also significantly increased from day 3 to day 5 in these mice. Histologically, cellular increase was observed in both the sinusoid of the liver and the red pulp of the spleen. Almost all cells that increased in the sinusoid of the liver were positive for F4/80, and cells positive for PCNA also increased. These changes were observed in mestranol-administered mice with or without spleen. In the morphometrical study, the peak number of F4/80 positive cells in the liver was observed from day 5 and day 7 in normal and splenectomized mice administered mentranol, respectively. On day 5 and day 7, the number of F4/80 positive cells observed in the mestranol-administered splenectomized mice was significantly higher than that in mestranol-administered normal mice. The peak number of PCNA positive cells in the liver was observed on day 3 in mestranol-administered splenectomized mice, and on day 5 in mestranol-administered normal mice. The number of PCNA positive cells was significantly higher on day 3 in splenectomized rather than normal mice administered mestranol. These results suggest that the liver disturbances observed in women taking estrogen for a long time may be closely related to local proliferation of Kupffer cells in the sinusoid.

The effect of estrogen on episodic secretory patterns of luteinizing hormone-releasing hormone (LHRH) and luteinizing hormone (LH) in hypergonadotropic hypogonadism

The secretory dynamics of plasma luteinizing hormone-releasing hormone (LHRH) and serum luteinizing hormone (LH) were studied in three hypogonadal women before and after chronic administration of mestranol. Blood samples were obtained through an indwelling iv line every 15 min over 3 hours, and plasma levels of LHRH and LH were measured by radioimmunoassay. LHRH and LH pulses were defined as rising from nadir to peak that exceed 2 times the intraassay coefficient of variation. All patients showed pulsatile LHRH and LH release before mestranol administration. The mean LH levels (89 +/- 20 mIU/ml) and pulse amplitude (33 +/- 14 mIU/ml) were significantly reduced after mestranol administration. On the other hand, the mean LHRH levels (1.87 +/- 0.49 pg/ml) and pulse amplitude (0.92 +/- 0.41 pg/ml) did not change significantly after mestranol administration. Pulse frequency (2 approximately 3 times/3 hrs) of LHRH and LH did not change after mestranol administration. These data show that the chronic administration of estrogen to such patients cause a decrease in mean LH levels and amplitude of LH pulse without a decrease of pulsatile LHRH secretions. These results suggest that the chronic negative feedback action of estrogen on episodic LH release in women may be at the level of the pituitary gland and estrogen may change the pituitary sensitivity to LHRH.

Effects of prenatal administration of mestranol and two progestins on testosterone synthesis and reproductive tract development in male rats.

The oestrogen mestranol (0, 0.01, 0.1 mg/kg body weight per day) and the progestins medroxyprogesterone-acetate and norethisterone (0, 2, 20 mg/kg body weight per day each) in sesame oil were intubated intragastrically daily during gestational days 14.5 through 19.5 to pregnant rats. Males were studied as 20.5-day-old foetuses and 4-month-old adults for serum testosterone and LH concentrations, in vitro testosterone synthesis, anogenital distance (foetuses only) and testes, seminal vesicle and ventral prostate weights. Administration of 0.1 mg mestranol decreased by 35 to 70% basal and LH-stimulated testosterone synthesis by both foetal and adult testes in vitro (P less than 0.01). Foetal body weights (P less than 0.05), but not anogenital distances, were significantly decreased. Testosterone content in adult sera was reduced significantly (P less than 0.05) to less than 50% of control. Testes, ventral prostate, seminal vesicle and epididymal weights were unaffected by treatment. Medroxyprogesterone acetate or norethisterone administration did not alter testes endocrine function in foetal or adult offspring. In a small number of rats, pregnant for 10.5, 14.5 or 18.5 days, [3H]ethinyloestradiol was intubated and foetal and placental tissue examined for appearance and content of radioactivity. Radioactivity was detected in 10.5, 14.5 and 18.5 days old placentas, and 14.5 and 18.5 days old foetal liver, gonads and external genitalia. With [3H]medroxyprogesterone acetate, radioactivity was localized in 14.5 day placenta and foetal tissues. Thin-layer chromatographic analysis showed most of the activity to migrate as authentic ethinyloestradiol or medroxyprogesterone acetate. These results demonstrate inhibition of testicular testosterone synthesis by mestranol, presumably by being transferred across the placenta and acting in the foetus.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of captopril and bradykinin on arterial pressure in rats with mestranol-induced hypertension

Hypertension was produced in 20 female rats by the oral administration of mestranol for 6 months; 20 control rats were not given mestranol during this time. The i.v. infusion of the angiotensin converting enzyme inhibitor, captopril, into 10 conscious mestranol-treated rats reduced mean arterial pressure from 139 ± 2 (SEM) to 130 ± 2 mm Hg (P < 0.01); mean arterial pressure in 10 conscious control rats averaged 123 ± 3 mm Hg, and was not significantly changed during captopril infusion. Injections of bradykinin at 0.1, 0.3, and 1.0 ug/100 gm body weight decreased mean arterial pressure significantly more in 10 mestranol-treated rats than in 10 control rats. These results suggest that the ability of captopril to lower arterial pressure in rats with mestranol-induced hypertension may be due, at least in part, to the action of captopril to enhance bradykinin.

Mestranol-induced hypertension: Characterization of cytochrome P-450 dependent catechol estrogen formation in brain microsomes

An animal model of estrogen-induced hypertension was used to study the effects of chronic administration of the synthetic estrogen mestranol on cytochrome P-450 content and catechol estrogen formation in brain microsomes. Cytochrome P-450 content of brain microsomes from untreated female rats in estrus was 0.034 nmole/mg protein and the dithionite-reduced carbon monoxide absorbance peak (γ max) was 452 nm. Catechol estrogen formation in brain microsomes was optimal in the presence of both NADPH and NADH cofactors with an apparent K m value of 71 μM for 17β-estradiol substrate. Brain microsomes from animals in estrus and diestrus were compared, and no significant differences were observed in cytochrome P-450 content, or in the apparent K m and V max values of catechol estrogen formation. Administration of mestranol, 15 μg biweekly, for 3–4 weeks resulted in a significant increase in systolic blood pressure in unanesthetized female rats. Mestranol treatment was not associated with a change in microsomal cytochrome P-450 content or the spectral γ max. At 10 μM substrate concentration, catechol estrogen formation in microsomes from mestranol-treated animals was increased 2- to 3-fold, with enzyme activity being expressed either per mg protein or per nmole cytochrome P-450. In contrast, no difference was observed between groups when enzyme activity was measured at 100 μM substrate concentration. These data suggest that chronic administration of a synthetic estrogen can regulate the enzyme system involved in formation of brain catechol estrogen metabolites, a mechanism which may alter the biological impact of the parent steroid.

Plasma levels and pharmacokinetics of ethynyl estrogens in various populations: II. Mestranol

A total of 89 adult women from Nigeria, Singapore, Sri Lanka, Thailand and the USA were given single oral doses of mestranol ranging from 50 to 100 μg; the subsequent plasma levels of free mestranol and ethynylestradiol were determined by radioimmunoassay, and certain pharmacokinetic parameters calculated. Mestranol is absorbed very rapidly from the digestive tract and maximum plasma levels are attained in 1–4 hours, the majority at 1–2 hours. Detectable levels of mestranol are present 24 hours postingestion in 58% of subjects after a 50 μg dose and in 79% after a 100 μg dose. At all dose levels, highest plasma mestranol levels and area under the curve of plasma levels (AUC) occurred in Sri Lankan women, and lowest plasma levels in Nigerian women, even when corrected for body surface differences. Plasma levels of ethynylestradiol derived from mestranol rose earlier and reached higher values than those of mestranol itself in every locality. The plasma levels of ethynylestradiol derived from mestranol were lowest and total body clearance highest in the Nigerian group at all dose levels; total body clearance was similar in the other localities. The AUC of plasma levels of ethynylestradiol was compared, by locality, on a dose-for-dose basis for the administration of mestranol or of ethynylestradiol itself. There was no significant difference between the two drugs, except at one dosage level in Thailand. These findings suggest that ethynylestradiol and mestranol are similar in terms of AUC; a higher peak and a faster decline are observed after administration of the former compound. Substantial differences between localities in the clearance rates of mestranol are suggested; whether these are due to differences in distribution or rate of metabolism or to differences in absorption remains to be determined.

Effects of mestranol on blood pressure and norepinephrine in young normotensive and genetically hypertensive rats

1. Increases in blood pressure (B.P.) were found in 3-month-old normotensive and genetically hypertensive (GH) rats following administration of mestranol. 2. In normotensive rats a depletion in hypothalamic and adrenal NE accompanied the increase in B.P. which was found at the end of 3 weeks of drug treatment. 3. In GH rats, after an experimental period of 3 weeks, a decrease in cardiac NE occurred, while at the end of 6 weeks, a depletion in adrenal NE which was accompanied by the increase in B.P., was found.

The Stein-Leventhal Syndrome: a Neuropituitary Disorder?

Hypothalamic and pituitary gonadotropin function and responsiveness in four patients with well-documented Stein-Leventhal syndrome were studied. All patients were of reproductive age and had had menstrual disorders since menarche. Estrogen production was assessed by measuring the circulating levels of immunoreactive estradiol, by vaginal smears, and by the progestogen-induced menses test. Gonadotropin function was evaluated by measuring the serum levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in blood samples obtained at 15-minute intervals for 150 minutes. Pituitary gonadotropin reserve and responsiveness were studied by giving an intravenous bolus of synthetic LH-releasing hormone (LH-RH) and measuring the circulating gonadotropin levels before and after the injection. Estrogen effects upon pituitary gonadotropins were studied by the administration of mestranol. Attempts to induce ovulation were made by using clomiphene citrate and repeated administration of LH-RH. The serum LH levels were persistently higher than those observed during the follicular and luteal phases of normal menstruating women, while the levels of FSH were within the normal range for the midfollicular phase. Episodic serum LH fluctuations of great amplitude were noted, while very few serum FSH oscillations were observed. Following the LH-RH bolus, a significant increase in serum LH (4.0-fold) and slight changes in serum FSH values were observed (1.8-fold). Mestranol administration neither suppressed the circulating levels of LH nor modified the levels of FSH. Administration of the antiestrogen resulted in successful induction of ovulation in all cases, while LH-RH therapy failed to induce ovulation. These data are interpreted as demonstrating a selective, elevated, pituitary LH release in basal and poststimulatory conditions with normal pituitary FSH function and responsiveness. It is suggested that the neuropituitary estrogenic (androgen-derived?) environment may play an important role in the nature of this disease. A close similarity of this syndrome to the estrogen-induced hypothalamic virilization phenomenon in the rat is discussed.

Influence of the oral contraceptive, menstranol, on drug-metabolizing enzymes of female rats in thiamin-supplemented and deficiency states.

The administration of a diet deficient in thiamin results in elevated hepatic microsomal activity of aniline hydroxylase, ethylmorphone demethylase, NADPH cytochrome c reductase and cytochrome P-450 when compared to similar female rats fed diets supplemented with thiamin. Accompanying these differences in enzyme activity are increased concentrations of microsomal docosahexaenoic acid and arachidonic acid. Binding of aniline to microsomes from rats fed high levels of thiamin is decreased due to a decrease in cytochrome P-450. On the other hand, the binding of ethylmorphine to P-450 is decreased by feeding high levels of thiamin. The daily administration of mestranol enhanced ethylmorphine and aniline metabolism to a greater extent in rats fed thiamin-rich diet than in rats fed thiamin-deficient diet or laboratory chow. This treatment did not increase cytochrome P-450 cytochrome c reductase or microsomal protein nor does it appear to affect the binding of aniline to cytochrome P-450. Ethylmorphine binding is generally decreased by this treatment. Alterations in the Michaelis constants for these reactions were limited to an increase in the Ks for aniline in pair-fed animals and in the Ks for ethylmorphine in thiamin-deficient rats receiving 1.0 mg mestranol per day.

The metabolic clearance rate and urinary excretion of oral contraceptive drugs: II. Mestranol

Mestranol, a synthetic estrogenic drug, was administered to seven normal females and the rate of disappearance of the drug and its metabolites from the plasma was followed. In addition, urine was collected over the 5 days following injection and analyzed for daily excretion rates and for types of conjugation of mestranol metabolites. The rate of clearance of mestranol from the plasma was rapid with an average metabolic clearance rate (MCR) of 1,247 L. per day. Following these initial determinations, subjects were placed on a commercial oral contraceptive preparation containing 80 μg of mestranol per day. Each subject completed six cycles of this drug preparation; after completion, the MCR and urinary excretion rate of the drug were redetermined. Statistical comparisons of the MCR and urinary excretion rates made before and again after the six cycles of the drug show that prolonged mestranol administration had no effect on the rate of clearance (MCR), the rate of urinary excretion, or the mode of conjugation of mestranol or its metabolites.

387. Steroid induced changes in the release of LH and FSH after LH-RH administration in human beings

It has been postulated that steroids may modify the release of gonadotropins after LH-RH administration. Before and after mestranol testosterone clomiphene and prednisone administration 100 mcg iv of synthetic LH-RH (Hoechst) was rapidly infused. Serial plasma samples were obtained for LH and FSH radioimmunoassay. 4 hypogonadal patients with high basal gonadotropin levels received weakly increasing doses of mestranol (40-480 mcg) or testosterone propionate (5-60mg). LH-RH was given before each increase in dose. 9 healthy male subjects received 200 mg of clomiphene citrate for 10 days and 6 patients 150 mg of prednisone daily for 5 days. Progressive decrease of both gonadotropins were observed following the initial administration of mestranol and testosterone. LH-RH administration continued to reduce FSH release but LH release showed 2 types of response: initial decrease at low dosage and marked increase with higher doses. Clomiphene administration magnified the basal response. Prednisone-treated patients showed no significant changes. From these results it can be concluded that one individual may release variable amounts of LH and FSH depending on the different levels of steroids administered. This emphasizes the complex interaction between LH-RH and sex steroids at pituitary level.(FULL TEXT)

Contraceptive steroids: Modifications of carbohydrate and lipid metabolism

Although chronic ingestion of contraceptive steroids may produce a variety of metabolic changes, their effect on carbohydrate and lipid metabolism appears to be related to the age and predisposition to metabolic disease of the recipient and to the amounts and relative proportions of estrogen, nortestosterone, and progesterone derivatives in the birth control preparations employed. In healthy intact young women, the separate administration of either synthetic estrogens or progesterone derivatives or nortestosterone derivatives does not consistently alter glucose or insulin metabolism as assessed by serum glucose or insulin concentrations after overnight fasting or following glucose administration. However, the concurrent administration of mestranol or ethinyl estradiol with a nortestosterone derivative produces deterioration of glucose tolerance, which is not observed during concurrent synthetic estrogenprogesterone derivative administration. The changes in glucose tolerance produced by birth control pills are superimposed on the deterioration of glucose tolerance observed with aging. Also, women with borderline pancreatic insulin reserve appear more likely to develop abnormal glucose tolerance than normal women when challenged with contraceptive steroids, although deterioration of glucose homeostasis does not seem to occur frequently in insulin-requiring diabetics using birth control pills. Concurrent mestranol-nortestosterone derivative administration generally enhances the peripheral circulating insulin response to a glucose challenge in normal women and in rhesus monkeys, whereas the individual contraceptive steroids and concurrent synthetic estrogen-progesterone derivative administration generally produce little measurable effect on circulating insulin concentrations. However, the rhesus monkey is not an ideal model for studying the metabolic effects of contraceptive steroids because its glucose tolerance improves with concurrent mestranol-norethindrone administration. No other suitable animal model has been developed for studying the effect of contraceptive steroids on carbohydrate metabolism. Estrogen administration leads to increased fasting serum triglyceride concentrations. Nortestosterone derivatives (but not 17 α-acetoxyprogesterone derivatives) counteract the hypertriglyceridemic effect of estrogens. Both of these effects appear to be dose-related. Contraceptive steroid-inducd hypertriglyceridemia primarily appears to be due to increased hepatic production of triglycerides, although plasma clearance of the triglycerides also seems to be reduced by the estrogen component of contraceptive steroids. Changes in serum cholesterol concentrations produced by birth control pills are small and are related to the age of the patient. In women over 40, total serum cholesterol concentrations decrease during birth control pill use; in women under 40, they increase. When increases in total serum cholesterol concentrations occur, they appear to be related to the amount of nortestosterone or progesterone derivative ingested. The possible relationship of these complex contraceptive steroid-induced changes in carbohydrate and lipid metabolism to the long-term development of cardiovascular diseases is discussed, although the mechanisms involved remain obscure.

Plasma proteins after use of estrogen and-or progestogen in women with and without recurrent cholestasis during previous pregnancies.

The authors report the concentration of some plasma proteins after administration of mestranol and/or chlormadinone in 19 controls and 43 women who had had recurrent cholestasis during previous pregnancies. Mestranol alone and combined with chlormadinone acetate induced enhanced βIC-globulin and fibrinogen and depressed cold insoluble globulin level. The changes were, if anything, more marked in women with cholestasis during an earlier pregnancy. No change of antichymotrypsin or C-reactive proteins was found in any of the groups. Chlormadinone acetate alone produced no apparent alterations in the plasma protein pattern.

Effect of estrogens on ascorbic acid in the plasma and blood vessels of guinea pigs

The administration of mestranol to guinea pigs resulted in a marked reduction of the ascorbic acid concentration in their blood plasma and an even greater reduction in the ascorbic acid concentration in their blood vessels. Theoretically, these changes could lower the negative potential of the blood vessel wall and might therefore predispose to intravascular thrombosis.

Failure of mestranol alone or with norethynodrel to prepare for the generalized Shwartzman reaction in rats

Female rats were given mestranol alone or in combination with norethynodrel for one or 2 months prior to intravenous injection of endotoxin. None of the drug-treated animals showed histologic evidence of the generalized Shwartzman reaction as did pregnant rats. Serial platelet counts and profibrinolysin levels indicate that intravascular coagulation in response to endotoxin is minimal in the treated groups as compared with the pregnant animals. Therefore, it may be concluded that administration of mestranol alone or in combination with norethynodrel does not potentiate the generalized Shwartzman reaction in rats as does pregnancy.

Effect of an oral contraceptive on spontaneous running activity of female rats.

Dietary administration of Enovid® at 7.5 mg/kg of food for 11 days to intact female rats reduced their cyclic running activity and induced changes in vaginal cytology characteristic of estrus. There was a linear relationship between the logarithm of the dose of Enovid® ingested and the percent decrease in running activity from the average observed before drug administration. Dietary administration of norethynodrel, one of the two components of Enovid®, at 7.5 mg/kg of food for 21 days decreased the magnitude of the running activity slightly but failed to affect its cyclic nature. In contrast, dietary administration of mestranol at either 0.125 or 0.54 μg/kg of food for 24 and 29 days respectively reduced running activity and increased the length of the estrus cycle. Hence, the effect of Enovid® on the running activity of intact rats is probably attributable to the mestranol contained in it. An apparent escape from the effects of chronic administration of mestranol on spontaneous running activity was observed from 20 to 30 days after beginning treatment with either dose used. The escape was characterized by a reinitiation of cyclic running activity during chronic drug administration. Elucidation of the mechanism of the escape phenomenon will require further study.

Effect of parenteral steroids on induction of genital tumors in mice by 20-methylcholanthrene

Insertion of 20-methylcholanthrene–impregnated thread into the cervical canal of intact female B6AF1 mice was found to induce squamous carcinomas of the cervix or vagina or adenoacanthomas of the uterus in 29, 14, and 15 per cent of the mice within a period of 15 weeks. Simultaneous subcutaneous implantation of pellets containing norethynodrel, progesterone, or the synthetic progestin 3β-17α-diacetoxy-6α-methyl-pregn-4-en-20-one (BL 141) increased the total number of genital tumors observed. Implantation of pellets containing mestranol alone had no significant effect on the total number of tumors induced by methylcholanthrene. Implantation of mestranol in addition to the other compounds studied failed to inhibit the tumor promoting effect of the progestational agents. Administration of either progesterone or BL 141 alone increased the percentage of squamous cell carcinomas of cervical origin. This effect was inhibited by simultaneous implantation of mestranol. Administration of mestranol and/or norethynodrel increased the percentage of adenoacanthomas arising from the lower uterine glands. This effect of mestranol was not inhibited by simultaneous administration of either progesterone or BL 141.

Studies on the site of action of oral contraceptive steroids. I. Effect of antifertility steroids on plasma LH levels and on the response to luteinizing hormone-releasing factor in rats.

ABSTRACT The effects of ovulation inhibiting steroids on plasma luteinizing hormone (LH) levels and on the response to LH-releasing factor (LRF) were studied in an attempt to learn whether these compounds exert part of their effect on the pituitary or on the hypothalamus. Ovariectomized rats were injected subcutaneously for 5 days with estrogens, progestins, and combinations thereof. Plasma LH levels were measured by the ovarian ascorbic acid depletion assay. The progestins: chlormadinone acetate (0.05–10 mg), ethynodiol diacetate (2 mg), norethynodrel (20 mg), norethindrone (5 mg), medroxyprogesterone (10 mg), norgestrel (1 mg) and dimethisterone (2.5–20 mg) given daily in doses indicated significantly depressed plasma LH. Administration of mestranol (0.001–0.2 mg) and ethinyl estradiol (0.1 mg) also significantly lowered plasma LH. The progestin/estrogen combinations: chlormadinone acetate/mestranol (C-Quens, 10.4 μg), ethynodiol diacetate/mestranol (Ovulen, 5.5 μg), norethynodrel/mestranol (Enovid, 12....