Effect of morphine sulfate on protein synthesis in rat brain was evaluated in tolerant and nontolerant rats. Male Wistar-Lewis rats were randomly distributed to control, tolerant and nontolerant groups. The rats were made tolerant by giving morphine sulfate (25 mg/kg b.i.d.) for either 42 or 84 days. An aqueous solution of 35S-methionine (0.5 muc/g b.w.t.) was administered i.v. Rats from each group were then sacrificed at 20 minutes, 1 hour and 2 hours after 35S-methionine. The 35S-activity in trichloroacetic acid-precipitated proteins and supernatant fractions from cortex, hypothalamus, putamen, corpus callosum, thalamus, cerebellum, kidney and liver was determined. In addition the amounts of label in lipid, saline-soluble and insoluble proteins of whole brain for tolerant and control rats were determined. The 35S-activity in brain proteins of non-tolerant rats did not differ from those of controls. The six brain areas in tolerant rats 1 and 2 hours after injection of 35S-methionine showed a 15 percent, then 30 percent increase in radioactivity (disintegrations per minute per milligram of protein). Proteins from putamen and corpus callosum had w/w 60 percent of the 35S-activity found in hypothalamus and cortex. There was no difference in 35S-activity in kidney and liver between control and tolerant rats. The radioactivity of the brain lipids was 2 percent of that found in proteins in both tolerant and control groups. Protein synthesis in morphine tolerant rat brain is significantly (P smaller than .01) increased as judged by incorporation of 35S-methionine.