Protein concentration increases in cerebrum and spinal cord during maturation of the rat were rather similar, but at all ages the spinal cord contained a higher protein concentration than cerebrum. Concentrations of cholesterol, galactolipids and phospholipids were higher in spinal cord than in cerebrum at all ages. Marked changes in the phospholipid composition in both tissues were also observed during development. The proportion of ethanolamine plasmalogens increased from 9 to 19% in cerebrum and from 18 to 29% in spinal cord. Choline phosphoglyceride proportions decreased from 58 to 33% in cerebrum and 54 to 25 % in spinal cord, while the proportion of sphingomyelin increased moderately throughout maturation in both tissues. The marked increase in concentrations of protein, cholesterol and galactolipids took place earlier in spinal cord than in cerebrum. The results suggest an earlier maturation of the myelin lipid composition in spinal cord than in brain, which is consistent with an earlier development of a phylogenetically older organ.