The influence of nutritional inadequacy during the active growth phase of the developing brain, coinciding with myelinogenesis, was examined in rat pups. Developmental profiles of enzyme activities involved in biosynthesis of myelin membrane lipids, and those of associated pathways which generate precursor compounds for such biosynthetic reactions, were followed as functions of age in major anatomical regions of the brain. It was observed that while galactosyltransferase and choline phosphotransferase activities were significantly diminished, the microsomal cytochrome reductases, which contribute to the process of fatty acid elongation and desaturation, were also lowered. An attempt at adaptative increase of enzyme activity, in response to the stress, was apparent in the case of glycerol-3-phosphate dehydrogenase, which could possibly explain, in part, the lower susceptibility of phospholipid biosynthesis to impairments induced by nutritional insufficiency.