Fatty acid content and composition of hepatic phospholipids was studied in rats during the fetal and postnatal period of development. Total phospholipid fatty acid content showed increases related to age including an elevation of saturated fatty acids such as myristic, stearic arachidic and lignoceric acids. Palmitic acid was unchanged in females and only slightly altered in males. Unsaturated fatty acids from total phospholipids were discretely increased, including linoleic, linolenic, arachidonic, eicosapentaenoic, docosapentaenoic and docosahexaenoic acids. Total fatty acid content of phosphatidylcholine and phosphatidylethanolamine fractions were also increased in relation to age in both sexes. The unsaturated components, however, showed a larger proportional increase. Major acyl components involved in these age-related changes were significantly increased, including linolenic and arachidonic acids; palmitic and docosahexaenoic acids were decreased. These data indicate that in rat hepatic phospholipids a selective increase in unsaturated fatty acids predominated during development with profound influences in the subcellular distribution of these moieties.