Abstract—The composition of sphingolipids and phospholipids of mouse brain during myelination was determined in normal animals and in mice with a genetically‐determined disorder of myelin formation. Myelination was normally characterized by a two‐fold increase in total phospholipids of brain, a four‐fold increase in total sphingolipids, and a six‐fold increase in cerebrosides. The Jimpy mutant, with defective formation of myelin in the central nervous system, demonstrated a marked deficiency of cerebrosides and a significantly lower content of total sphingolipids, without alteration of the composition of phospholipids. The increasing content of cerebrosides in the brains of the leucodystrophic mutant at the time in development when myelination is most active and the subsequent relative deficit suggest that the failure of myelin formation is not the result of a defect in biosynthesis of cer