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RU 486 is a synthetic steroid which has antiprogesterone and antiglucocorticoid activity. In order to determine the optimal dosage of this drug to terminate early pregnancy, we treated 106 healthy women with normal pregnancies by real time ultrasound examination whose gestational duration was less than 49 days from onset of last menses with either a medium or low dose treatment regimen. A total of 66 patients received the medium dose regimen (100 mg/day X 7 days). Another 10 patients received ergonovine (0.2 mg/day X 6 doses) on Day 4 of the same RU 486 treatment regimen. In the first group, 48 (73%) aborted successfully and, of the second group, 6 (60%) aborted. Eighty percent of the subjects in this group of 76 patients reported side effects of nausea and vomiting, heavy bleeding, severe menstrual cramps or headache. All these side effects were successfully treated with analgesic and antiemetic medication. The remaining 30 subjects were treated with a low dose regimen (50 mg/day X 7 days). Of these 30, 15 (50%) aborted; this incidence was significantly less (p less than 0.05). Following the medium dose treatment regimen, the AM cortisol levels were significantly elevated on treatment Days 4 and 8, as compared to baseline (p less than 0.001), although the mean levels were still within the normal range. With the low dose, there was a non-significant rise in AM cortisol values. Thus the rise in cortisol was significantly greater in the former group than the latter (p less than 0.05). With the medium dose regimen, the women who aborted had significantly lower (p less than 0.05) pretreatment mean B-HCG and progesterone levels than the group that failed to abort. Mean serum levels of RU 486 were not significantly different between the group who aborted and those who did not. RU 486 is a promising agent for termination of early pregnancy.