Abstract: Administration of gossypol acetic acid 80 mg/kg/day to rats on day 6～9 of gestation terminated early pregnancy. Progesterone 5 mg/day or hCG 25 IU/day was shown to block the antifertility effect of gossypol treatment.Rats ovariectomized on day 6 of pregnancy were injected daily with progesterone 10 mg/day and 1 μg/day until day 15. The fetuses were found to be alive on day 16 of gestation. When gossypol acetic acid 80 mg/kg/day was given to these rats, no antifertility effect was observed.Serum progesterone level of rats treated with gossypol acetic acid alone was found to be markedly lower than that of control animals. Intact rats were injected subcutaneously hCG with or without gossypol acetic acid on day 6～9 of gestation. Steroidogenic response of rat ovary to hCG was inhibited by gossypol acetic acid. It is postulated that the blockade of gonadotrophic effect of hCG may be the chief mechanism by which gossypol acetic acid interrupts early pregnancy in female rats.