The microsomal synthesis of docosahexaenoic acid (DHA) from radiolabeled eicosapentaenoic acid was tested in vitro using the livers of crab-eating monkeys. The test animals included 1 fetus (embryonic day 120), 2 neonates (full-term, embryonic day 165), and 2 adults. DHA was formed equally in the liver microsomes of each animal. Neonatal livers were found to contain higher percent of arachidonic acid than adult livers in both phosphatidylcholine and phosphatidylethanolamine. DHA contents remained constant in the liver phospholipids of all test animals.