Background: Aqueous extract of winter cherry (Physalis alkekengi L; family of Solanaceae) fruits (WCF) has long been recommended for fertility control by herbalists in Iran. The effect of this extract on lowering serum progesterone levels has been reported previously. Objective: To study the effects of WCF extract on the activities of ovarian 3β-hydroxysteroid dehydrogenase (3β-HSD) responsible for the synthesis of progesterone and 20α-HSD responsible for its degradation in rats. Methods: One ml aliquots of the aqueous extract of WCF (containing 400 mg of dried extract) were intraperitoneally injected for 8 consecutive days to rats from day 8 of pregnancy. Rats were then sacrificed 21 days and 11 hours after the day of observing sperm positive vaginal smears. Blood was collected for the determination of serum progesterone and the total and the live numbers of embryos were counted. Ovaries were also used for the measurement of the activities of 3β- and 20α-HSD. Results: The extract of WCF decreased the ovarian 3β-HSD specific activity by 47%, serum progesterone concentration by 30% and the number of live embryos by 67%, but it had no effect on the specific activity of ovarian 20α-HSD. Conclusion: The aqueous extract of WCF, containing steroidal compounds with known estrogen antagonistic properties, probably interferes with the function of estradiol in inducing ovarian 3β-HSD synthesis. It may also contain components which inhibit this enzyme, thus reducing progesterone synthesis that is required for maintaining pregnancy. Such natural compounds, if purified, might be beneficial for control of fertility.