Midtrimester abortion was successfully induced in 117 of 120 patients with serial intramuscular injections of 15(S)-15-methyl-prostaglandin F2alpha (15-me-PGF2alpha). The mean abortion time was 14.12 hours, and parous patients aborted in a mean of 12.85 hours-significantly faster than nulliparous patients who aborted in a mean of 15.24 hours. Ninety-four per cent of the 117 successfully induced abortions occurred in less than 24 hours and 46 per cent in less than 12 hours. Uterine activity was monitored and analyzed in nine patients. Uterine response to a single intramuscular injection of 100 mug of 15-me-PGF2alpha was characterized by the appearance of low-amplitude, high frequency contractions and a rapid increase in baseline intrauterine tonus. A high level of uterine activity, 900 Montevideo Units, was observed within 30 minutes of the first intramuscular injection of 15-me-PGF2alpha. This activity was not maintained and decreased by approximately 30 per cent at the time of the second injection at 1 hour. It was not until 6 hours of 15-me-PGF2alpha therapy that activity stabilized at approximately 500 Montevideo Units. Even though all patients were premedicated with antiemetic and antidiarrhea agents, 68 of 120 patients experienced gastrointestinal side effects related to the 15-me-PGF2alpha administration. Vomiting was the most prevalent side effect, occuring in 65 patients, but the episodes were not severe, were well tolerated by the patients and did not necessitate the termination of prostaglandin administration in any of the patients. In this study abortion was successfully induced between weeks 9 to 27 of gestation. It was observed that patients with gestations of 16 weeks or less aborted significantly faster than patients with gestations of 17 weeks or more, which indicates that this method is highly effective in the induction of abortion within the "gray zone", 12 to 16 weeks of gestation.