The fatty acid composition of triglycerides and phospholipids from rat liver was determined throughout the period of growth in the rat. Major changes in the triglyceride fatty acid composition were observed during the period studied. The triglycerides from fetal and newborn rats contained only a small percentage of polyunsaturated acids compared with suckling and weanling rats. During the suckling phase the liver triglycerides were rich in long chain polyunsaturated acids such as 20:4, 20:5, 22:5, and 22:6; once the pups were weaned, the percentage of these acids in the liver triglycerides fell. In these experiments, 18:2 and 18:3 were the only polyunsaturated acids in the maternal diet. However, the stomach contents of the suckling pups contained 18:2 and 18:3, as well as the long chain polyunsaturated acids. Radioactive 18:3 and 22:6 were fed to suckling pups, and the results suggested that the LCP in the rat liver triglycerides during the suckling period were derived from the long chain polyunsaturated acids in the dam's milk, rather than by synthesis from either 18:2 or 18:3 within the pups.