A development study of major lipid fatty acids in human brain myelin was undertaken and compared to those in cerebral white matter of the same region. The myelin was isolated from 23 subjects at ages from newborn to old age. The proportions of cholesterol and galactolipids increased in myelin during the first 6 months of age and up to 2 years of age in cerebral white matter. During the same periods the individual phospholipids also showed marked variations. Serine phosphoglycerides and especially sphingomyelins increased, and choline phosphoglycerides decreased. The fatty acid patterns of ethanolamine phosphoglycerides (EPG) and sphingomyelins underwent the largest maturation changes. The proportions of saturated fatty acids in EPG diminished rapidly with a corresponding increase of monoenoic acids. Fatty acids of the linoleic acid series showed a peak between 4 months and 12 months of age, and then their proportion slowly diminished to old age. The fatty acid changes in serine phosphoglycerides were much less pronounced than in EPG but of similar type. In sphingomyelin the proportion of saturated long-chain fatty acids diminished while the proportion of monoenoic acids increased--this increase continued at least to the age of 15 years. The same fatty acid changes occurred in cerebrosides and sulfatides as in the sphingomyelins, but they were less pronounced. The fatty acid changes during development were much more pronounced in white matter than in myelin but already from 1-2 years of age the lipids of myelin and white matter had the same patterns--in the galactolipids from 2 months of age. The individual variations of the lipid fatty acid patterns were small except for at the youngest ages and the variations found for this period might depend on the difficulties in determining the gestational age.