Myelin was isolated from rat brains during development by a procedure giving fractions of constant purity at all ages. The lipid composition of these fractions and of whole brains of littermates was determined. The amount of myelin recovered per brain was a nearly linear function of the logarithm of age from the youngest (15 days) to the oldest (425 days) animals studied. With the exception of the earliest age point, the isolated myelin accounted for approximately 40 per cent of total brain galactolipid, evidence that a constant fraction (calculated to be 60 per cent) of myelin was recovered at all ages. Although the lipid‐protein ratio of the myelin was constant with age, marked changes were seen in the amounts of cerebroside, sulphatide, phosphatidylcholine and desmosterol. The total galactolipid increased from 21 per cent of the total lipid at age 15 days to about 31 per cent at maturity. Phosphatidylcholine decreased from 17 to 11 per cent during the same period. Desmosterol decreased from 2.5 per cent of the total sterol to 0.2‐0.3 per cent. All of these changes were complete between 2 and 5 months of age; no other ‘lower phase’ lipids showed significant changes with age. Although qualitatively similar to those reported by others, the changes differed in magnitude, with more stability in the levels of cholesterol and phosphatidalethanolamine with development. A sensitive indicator of the maturation of myelin was the mole ratio galactolipid/phosphatidylcholine, which varied from 1.2 at age 15 days to 2.8 at maturity. The maximum rate of myelination occurred at 20 days of postnatal age when myelin was deposited at the rate of 3.5 mg day⁻¹ brain⁻¹. However, at this age the rat brain had only 15 per cent of its eventual complement of myelin. The rate of accumulation of cerebroside in the whole brain paralleled that of myelin, and was the only lipid to show this relationship. Myelin deposition appeared to be almost solely responsible for the continued increase in brain weight after about 100 days of age.