Changes observed as a function of chick age in fatty acid composition of lipids from liver microsomes were considerably small, while the unsaturation index increased throughout postnatal development. Supplementation of the diet with 2% cholesterol from hatching produced a significant decrease in the levels of palmitic acid and a clear increase in those of polyunsaturated fatty acids. Maximum effects were attained on day 19 of treatment. Alterations in the fatty acid composition were more pronounced after short-term (48 h) cholesterol feeding. Administration for 48 h of a standard diet to chicks fed a cholesterol diet for 10 days from hatching restored the levels of fatty acids to those of the controls. However, when cholesterol feeding was prolonged for 24 days from hatching, no effect was found after the same treatment. Suppression of the cholesterol diet for 48 h in animals cholesterol fed for 48 h had no effect in 12-day-old chicks while the change to a standard diet produced a reversion of the effect of cholesterol feeding in 26-day-old animals.