A complete interceptive action on pregnancy was shown after ig epostane (Epo) 48 and 96 mg/kg on d 10 of pregnancy in rats. ED50 (95% fiducial limits) of Epo was 20.7 (16.3-26.3) mg/kg. Epo 48 mg/kg ig on d12, 14 and 16 of pregnancy lowered rat plasma progesterone concentrations (P less than 0.05) and plasma corticosterone on d 16 of pregnancy (P less than 0.05). Epo 35 mg/(kg.d) was given ig on d 50-54 of pregnancy in rhesus monkeys. Vaginal bleeding was seen in all of the 6 treated monkeys. Complete expulsions of fetal and placental materials occurred in 5 of the treated monkeys, among which 2 aborted on d 53 and the other 3 on d 54 of pregnancy. Plasma progesterone concentrations of the treated monkeys were lowered on d 52, 53 and 54 of pregnancy (P less than 0.05) and plasma estradiol on d 51, 52, 53, 54 and 55 (P less than 0.05), while there were no significant differences in plasma hydrocortisone. Epo may be a valuable supplement to the existing methods of female contraception.