Placentas were obtained from 35 healthy women whose pregnancy was terminated in the first trimester. Seventeen of them had no prostaglandin medication whereas in 18 women 500 micrograms of sulprostone was administered for cervical softening 12-14 h prior to the termination of pregnancy. Concentrations of progesterone and 20 alpha-dihydroprogesterone were determined from the extract of the placenta by liquid chromatography and those of estrone and estradiol by a radioimmunoassay. Placental progesterone concentration (6,829 +/- 469 ng/g wet weight, mean +/- SEM) was higher in the sulprostone group than in the control group (5,070 +/- 539 ng/g, p less than 0.02), but the mean concentrations of 20 alpha-dihydroprogesterone, estrone and estradiol were similar in both groups. Thus the small dose of sulprostone used in this study did not result in any great changes in placental steroidogenesis which could bring about the initiation of abortion. The increase of placental progesterone may, however, indicate that uterine contractions and impaired uteroplacental blood flow secondarily decreased the direct transfer of steroids from the placenta to the myometrium.