In this report data are summarized on changes in the quantity of proteolipid protein (PLP), its amino acid composition, and the lipid moiety of these lipid-protein complexes in rat brain during postnatal development. In all three parts of the central nervous system (CNS) studied (cerebral hemispheres, medulla oblongata and spinal cord) the main pattern of PLP accumulation is on the whole similar. PLP content is very low in the newborn, and it increased 12 to 20-fold during development. The highest rate of PLP accumulation is observed in the period from 10 to 30 days after birth. Against the background of general similarity the concentration of some amino acids such as lysine, proline, tyrosine in PLP somewhat increased during development, while that of aspartic acid, glutamic acid, glycine, and leucine decreased. Soluble proteolipid complexes, purified to various degree from lipids were isolated from brain of rats of different ages. As compared with the original lipid extracts from which they were obtained, the crude and especially purified proteolipids in all the animals studied were enriched in acidic phospholipids (PhL). This prevalence of acidic PhL increased with age. During the development in phospholipid moiety of proteolipids (PL) the content of phosphatidyl serine, sphingomyelin and mainly diphosphatidyl glycerol increases and that of phosphatidyl inositol and especially phosphatidyl choline decreases. The concentration of acidic PhL more tightly bound with PLP appreciably increases with age. Most of these changes occur mainly during the second decade after birth.