The effects of several food flavonoids on the activities of some drug-metabolizing enzymes in rat-liver microsomes were determined in vivo. When quercetin, chrysin, hesperetin and (+)-catechin were injected ip at doses of 150 mg/kg body weight/day for 3 days, quercetin and chrysin stimulated aminopyrine demethylase. When quercetin, rutin, hesperetin and (+)-catechin were fed as components (0 . 1%) of purified diets administered to rats for 4 wk, rutin and (+)-catechin increased microsomal cytochrome b5 levels and quercetin and (+)-catechin increased aminopyrine demethylase activity. On the other hand rutin, quercetin and hesperetin inhibited NADH-cytochrome c reductase. Aniline hydroxylase was not affected by any of the flavonoids administered by either route. The results show that each flavonoid affects different components of the monooxygenase system. The fact that some flavonoids give different effects according to the route and/or the duration of administration is discussed.