The effect of acute and chronic morphine treatment on the synthesis and turnover of 3H-leucine-labeled protein and 14C-choline-labeled phosphatidylcholine was measured in discrete regions of the rat brain. Chronic morphine treatment had the following effects on turnover. In the crude mitochondrial fraction of all brain regions studied, the turnover of 3H-protein was decreased. Microsomal 3H-protein turnover was increased in the cerebellum and hypothalamus and decreased in the cortex. The turnover of 14C-phosphatidylcholine was increased in the crude mitochondrial fraction of the brain stem, hypothalamus and diencephalon, but decreased in the cortex. In the microsomal fraction, the turnover of 14C-phosphatidylcholine was decreased in the cortex, brain stem and caudate nucleus, but increased in the diencephalon. Acute morphine treatment decreased 14C-phosphatidylcholine synthesis in the cortex (58 per cent) and cerebellum (49 per cent), but increased synthesis in the hypothalamus (95 per cent) and diencephalon (285 per cent). Acute morphine treatment decreased 3H-protein synthesis in the cortex (77 per cent) and diencephalon (73 per cent), but increased 3H-protein synthesis in the hypothalamus (55 per cent) and caudate nucleus (146 per cent). The relevance of these findings to current theories of narcotic tolerance and physical dependence development is discussed.