RU 486 is an antiprogestin which acts at the receptor level. In the present study the effect of this compound on uterine contractility and sensitivity during early pregnancy was evaluated in 10 patients. Five patients in the 6th to 7th week of pregnancy received 25 mg RU 486 twice daily for four days. On the fourth day of treatment, uterine contractility was recorded. The remaining five early pregnant patients were untreated and served as control. Withdrawal of progesterone locally by RU 486 treatment resulted in the development of a regular uterine activity which was in sharp contrast to the low level contractility pattern found in the untreated control patients. Also the sensitivity to prostaglandin increased following RU 486 treatment. The efficacy of a sequential therapy of RU 486 and the PGE analogue 16-phenoxy-tetranor-PGE2 methyl sulfonylamide for termination of early pregnancy was also studied. Thirty-four early pregnant women (duration of amenorrhea for up to 49 days) admitted to the hospital for termination of their pregnancy volunteered for the study. The patients received 25 mg RU 486 twice or four times daily for four days. In the morning of the fourth day of RU 486 treatment, a small dose (0.25 mg) of the PGE analogue was given as a single intramuscular injection. The combined treatment resulted in complete abortion in 32 patients (94%). One patient experienced an incomplete abortion and in one patient the pregnancy continued unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)