Both the cytochrome b5 level and NADH cytochrome b5 reductase activity in rat liver microsomes were increased 2-fold by repeated i.p. administration of 1.5 mmol/kg propylthiouracil (PTU) for 2 weeks, but neither the cytochrome P-450 level nor NADPH cytochrome P-450 reductase activity were affected by the treatment. Liver microsomes from PTU-treated rats showed a significant decrease in aminopyrine N-demethylation, but not in benzphetamine N-demethylation, aniline hydroxylation or 7-ethoxycoumarin O-deethylation. A single administration of the compound had no effect on any components of the system. In vitro, drug hydroxylation activities were not affected by PTU up to 1.0 mM. From the above evidence, repeated administration of PTU selectively induced cytochrome b5 and NADH cytochrome b5 reductase in rat liver microsomes.