Expression of the jimpy gene in heterozygous females was analyzed by measuring galactolipid synthesis in brain during early myelination. Sulfatide labeling in brains of heterozygous females at 13-14 days is decreased to 40-80% that of female littermates who do not carry the jimpy gene. The activities of ceramide galactosyl transferase and cerebroside sulfotransferase and levels of myelin basic protein were similarly depressed. Since the jimpy gene was maintained with the Tabby gene in these studies, the effect of the Tabby gene on these parameters was examined and found to have no effect. These biochemical findings indicate that myelination is retarded in the brains of heterozygous jimpy females during the second week of development. However, at 18 days and thereafter, sulfatide labeling is less reduced, suggesting that the oligodendrocytes in brain attempt to compensate, in agreement with morphologic studies which show that myelin is decreased in brain during early development, but appears normal in adult animals.