1. The effect of inhibition of cholesterol synthesis by a hypocholesterolaemic drug (AY-9944) was studied in rat brain during development. 2. At 2 weeks after administration of AY-9944 to young rats 7-dehydrocholesterol accounted for half the total sterol of myelin and other subcellular components. 3. At 4 weeks after injection of the drug 7-dehydrocholesterol had disappeared whereas the cholesterol content of myelin had increased by an equivalent amount. Our studies show that purified myelin has low 7-dehydrocholesterol reductase activity and suggest that 7-dehydrocholesterol is largely converted into cholesterol outside the myelin sheath. 4. Resultant cholesterol may be re-incorporated into myelin by an exchange process. 5. The metabolism of sterols in developing brain is discussed.

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