Myelin was isolated from the sciatic nerves of chicks of ages 18-day embryonic, 1-day, 4-day, 7-day post-hatch, and adult to study developmental changes in lipid composition of this structure. The yield of myelin increased throughout the early stages of development and the preparations were of high purity. Although the lipid content of the myelin did not change, significant changes took place in lipid composition during development. The most significant changes were a relative increase in cerebrosides, phosphatidalethanolamine and long-chain fatty acids of cerebrosides, and a relative decrease in the content of phosphatidylserine and phosphatidylethanolamine. A second fraction ("lower band") was obtained during the isolation procedure. This "lower band" was present at all developmental stages and layered consistently at the interface of 1.2 and 0.8 M sucrose on a discontinuous gradient. The quantity of this fraction did not change during development and it differed from myelin in electron microscopic appearance. Its lipid composition, which did not change, resembled that of 18-day embryonic myelin in its high phospholipid:cholesterol ratio and low galactolipid content. The enzyme, 2':3'-cyclic-nucleosidemonophosphate phosphodiesterase was found to be present in both the myelin and "lower band" fractions; however there was no enrichment of this enzyme in purified myelin.