The ascorbate-dependent lipid peroxidation in the microsomal fraction of at term placentas and fetal membranes were studied. In preliminary experiments, the optimum conditions for the measurement of this reaction were determined. Lipid peroxidation was significantly higher in the chorion and amnion compared to the placenta. In both fetal membranes, but not in the placenta, the reaction was enhanced by arachidonic acid and inhibited by indomethacin. The results indicate that the ascorbate-dependent lipid peroxidation in the fetal membranes is specific for prostaglandin synthetase activity and that these tissues are a major site of prostaglandin synthesis.