Two doses, 200 and 400 micrograms, of misoprostol, administered vaginally every 12 hours, up to four times, were tested in 101 and 133 healthy women, respectively, for interruption of pregnancies with 35 through 77 days of amenorrhea. The proportion of women who aborted increased with longer duration of treatment and was significantly higher with 400 than with 200 micrograms (66 versus 46 percent at 48 hours). Significance was maintained after controlling by age, body weight, parity, previous abortion and gestational age. Abortions were classified as incomplete or complete, according to the presence or not of embryonic tissue in the uterine cavity, diagnosed by vaginal sonography. Vacuum aspiration was carried out in all cases not classified as complete abortion 48 hours after the initiation of treatment, or earlier in case of persistent bleeding or woman's request. The possibility of increasing effectiveness by using higher dose, shorter intervals or longer duration of treatment is discussed.