Abstract
1. The turnover of lipids, of myelin and other brain subcellular particles has been studied in double-labelling experiments on intact rats. 2. Overall metabolism of brain mitochondrial lipids was three times slower than that of the liver. 3. Individual lipids of brain mitochondria and myelin were also separated and their metabolism was studied. 4. All myelin lipids examined undergo very slow turnover. Two pools of brain mitochondrial lipid were identified. The slowly metabolized lipids were cholesterol, cardiolipin plus phosphatidic acid and possibly sphingomyelin; the remaining phosphatides underwent more rapid turnover. 5. The possible significance of these results is discussed.