Abstract Tunicamycin caused a dose and time dependent decrease in cytochrome P-450 in rat liver. A dose of 50 μg/kg caused a decrease of about 50% in 72 hours. A similar decrease in the activities of rat liver microsomal aniline hydroxylase, aminopyrine N-demethylase and ethoxycoumarin O-deethylase were also seen after the tunicamycin treatment. Tunicamycin also suppressed food and water intake but the decrease in cytochrome P-450 was not related to these effects. NADPH cytochrome c reductase was not markedly decreased by tunicamycin. A decrease in cytochrome P-450 was also observed in cultured rat hepatocytes treated with tunicamycin. It decreased incorporation of [ 35S]-methionine into total proteins as well as into various cytochrome P-450 isozymes of rat hepatocytes. This indicates that a decrease in protein synthesis may be responsible for the tunicamycin-induced decrease in cytochrome P-450 and drug metabolism.