Thirteen women received a single oral dose of 600 mg mifepristone (RU486; Roussel-Uclaf, Paris, France) on day 0 to induce first-trimester abortion. Eleven women had a complete abortion, and 2 had a continuing pregnancy and were considered as failures. Plasma levels of human chorionic gonadotropin (hCG) increased from day 0 in all women, but dropped sharply after day 3 in women with successfully induced abortion. In these women, the levels of progesterone, 17 alpha-hydroxyprogesterone and estradiol-17 beta increased from day 0 to day 1 and then dropped. In the 2 failed inductions, the steroid levels remained virtually unchanged. In the complete abortions, the main prostaglandin F2 alpha metabolite, 15-keto-13,14-dihydro-PGF2 alpha, increased to a significant peak at day 6 followed by a significant fall to day 14. These findings supports a direct effect of mifepristone on the decidua, initiating prostaglandin synthesis. It is also suggested that there is a luteolytic effect of prostaglandin F2 alpha, as the levels of corpus luteum steroids started to fall before the drop in hCG.