Total fatty acid content of infant brain was determined to assess minimal fatty acid requirements for synthesis of structural lipids in brain tissue during the first 13 weeks of life. Fatty acid accretion rates were determined by regression analysis of tissue fat components at varying postnatal ages. Quantitative fatty acid analysis indicated that postnatal accretion of long-chain polyenoic fatty acids in cerebellum, frontal and occipital brain lobes initially appeared to lag behind the rate of increase in brain weight and brain fat content. After a 4-week period total long-chain fatty acids increased at a rate similar to the increase in brain weight. Accretion of individual fatty acids in cerebellum also reflected changes in tissue total fatty acid content. These developmental changes in fatty acid components of whole brain and cerebellum are quantitatively relevant to estimation of the minimal extrauterine fatty acid requirements of the human neonate.