Metabolism of [(35)S]sulpholipid was studied in rats of different ages after the injection of [(35)S]sulphate. 2. The turnover of sulphatide in brain myelin and subcellular fractions was followed for long periods. 3. A small fraction of adult myelin underwent rapid metabolism, the rest being relatively metabolically stable. 4. Fast turnover of brain microsomal sulphatide during development was related to the process of myelination. 5. Turnover of mitochondrial sulphatide was at a lower rate than that calculated for liver mitochondria. 6. The relevance of the results to the metabolism of the whole membrane is discussed.