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The effects of a progesterone antagonist ZK 98.734 (lilopristone) on implantation, early pregnancy, and midpregnancy were studied in the common marmoset, Callithrix jacchus jacchus. Treatment (5 mg/da intramuscularly for 3 consecutive days) on day (n = 8) after the midcycle peak in estradiol levels in mated animals induced a premature drop in plasma progesterone levels and shortened the ovarian cycle length. Treatment on day 20 (n = 5) or day 40 (n = 5) induced a drop in progesterone levels and decidual collapse. In three animals treated on day 40, vaginal bleeding was observed within 46 hours of the initiation of treatment. Treatment on day 80 resulted in expulsion of the fetuses with a mean induction abortion interval of 39 hours (range, 20 to 48 hours). The progesterone antagonistic effects of ZK 98.734 could be a result of the decrease in progesterone synthesis by the corpus luteum and/or placenta in addition to the interference with the progesterone binding to its cellular receptors in the target organ. Our study suggests that ZK 98.734 has potential for fertility regulation. Clinical trials for postcoital contraception, induction of menstruation, and early abortifacient effects are warranted.