The progesterone antagonist mifepristone (RU486), was given in mice once on different days of pregnant mare's serum gonadotrophin-human chorionic gonadotrophin (PMSG-HCG) treatment and its action upon the induction of ovulation studied. RU486 administered on the day after PMSG significantly reduced the ovulation rate. Ovulation was completely inhibited when the progesterone antagonist was given simultaneously with HCG, but RU486 administered 4 h after HCG treatment remained ineffective. The development of two-cell zygotes harvested on day 2 post-coitum from mice treated with RU486 on the day after the PMSG treatment was followed in vitro and showed a significant decrease in the number of embryos developing to blastocysts. These results favour the involvement of progesterone in the ovulation process, indicating a direct effect of this hormone at the ovarian level via a progesterone receptor-mediated action.