There have been conflicting reports on the effect of non-steroidal anti-inflammatory drugs on the abortifacient effect of prostaglandins. The efficacy of intra-amniotic 1.5 mg 15(S) 15 methyl PGF2 alpha (15 me PGF2 alpha) in terminating mid-trimester pregnancy in 10 subjects has been compared with the effect of the same medication given to 10 others who had had indomethacin 25 mg and 50 mg orally three and one hour before the PG administration. In the subjects who had the PG analogue only the injection-abortion interval was 17.0 hours. This was significantly shorter (P less than 0.01) than in those who had also received indomethacin (25.4 hours).