Microsomal fractions isolated from rat corpus striatum catalyze the oxidation of thiobenzamide to the sulfoxide. The rate of thiobenzamide sulfoxidation is 6.9 +/- 4.8 nmol(min)-1 (mg microsomal protein)-1. The reaction is inhibited by an excess of sulfur- and nitrogen-containing substrates for the microsomal flavin-containing monooxygenase. These inhibitors of thiobenzamide sulfoxidation include methimazole, cysteamine, and trimethylamine. Enzyme activity is also destroyed by treatment of the microsomal preparation at 60 degrees for 1 min. In parallel experiments, rat liver microsomes exhibit similar inhibition characteristics. The data indicate the presence in corpus striatum of a microsomal monooxygenase with catalytic properties of the hepatic microsomal flavin-containing monooxygenase.