Morphological, biochemical, and physicochemical studies of myelin subfractions were undertaken on the progeny of Sprague-Dawley rats fed diets containing lipids either extracted from yeasts grown on n-alkanes or from margarine. Myelin subfractions obtained from pooled brain homogenates of littermates by sucrose density gradient centrifugation at 7, 14, and 21 days postnatally were subjected to electron microscopy, sodium dodecylsulfate polyacrylamide gel electrophoresis and assayed for 2', 3' cyclic nucleotide 3'-phosphohydrolase activity (CNPase; EC 3.1.4.37). Additionally, surface pressure measurements were made of lipid monolayers derived from myelin subfractions, which were subsequently injected with myelin basic proteins. The myelin subfractions of test animals, when compared with those of controls, show an earlier increase in the specific activity of CNPase, the earlier appearance of low-molecular-weight proteins, and an increase in the affinity of basic proteins for lipids derived from the myelin light fraction. This biochemistry suggests the presence of a more mature myelin between 7 and 14 days in the experimental group. The morphological studies, however, do not seem to concur with the biochemical data. The observed changes are discussed in relation to the influence of dietary lipids on myelinogenesis.