The rate of incorporation of 3H-arachidonic acid and 14C-palmitic acid into 15 term amniotic membranes was studied during incubation of tissues in an artificial amniotic fluid medium. Phospholipids were extracted from the amnions, separated by thin-layer chromatography, and both the radioactivity and the phosphorus associated with each phospholipid species were determined. The distribution of amnion phospholipids was: lecithin, 47 per cent; phosphatidyl ethanolamine, 30 per cent; sphingomyelin, 7 per cent; phosphatidyl inositol, 5 per cent; lysolecithin, 4 per cent; and other phospholipids, 6 per cent. During incubation, the specific activities of the phospholipids became maximal after 2 hours and remained constant for at least the next 2 hours. Arachidonic acid was incorporated mainly into lecithin (53 per cent) and phosphatidyl ethanolamine (27 per cent) whereas palmitic acid was incorporated mainly into lecithin (70 per cent). Incorporation of fatty acids was 33 per cent higher in amnions obtained after vaginal delivery than in those obtained before labor. From the known fatty acid composition of amnion phospholipids, it is concluded that there is a high rate of turnover of arachidonic acid relative to palmitic acid in lecithin.