Free arachidonic acid was measured by gas liquid chromatography in amniotic fluid in 13 women during induction of midtrimester abortion. The abortions were induced by extraamniotic instillation of 0.1% Rivanol. Serial sampling of amniotic fluid were performed through a transabdominal catheter up to 22 hrs. Free, total arachidonic acid showed a significant increase from 26 +/- 8 ng/ml to 293 +/- 102 ng/ml at 22 hrs. The percentage of free arachidonic acid in total free fatty acids increased significantly from 2.2 +/- 0.5 to 6.1 +/- 1.6+ during the same time. The results suggest a selective release of arachidonic acid during Rivanol-induced abortion.