The majority of insecticides currently in use throughout the world belong to the class of the organophosphorus insecticides. Many of these compounds, such as the phosphorothioate insecticides, exert their mammalian toxicity only after undergoing metabolic activation by a variety of cytochrome P450 isoforms to produce their corresponding oxygen analogs (or oxons), which are potent inhibitors of the critical enzyme acetylcholinesterase. Of the many chemicals identified that can modulate cytochrome P450-dependent activities, the flavonoids represent some of the most unusual compounds in that they have been reported to both inhibit and stimulate certain activities. The present study was undertaken to determine if representative flavonoids (at in vitro concentrations of 1-100 microM) can alter the mammalian cytochrome P450-dependent biotransformation and acute toxicity of the phosphorothioate insecticide parathion. The flavonoids 5,6-benzoflavone, flavone, and quercetin had the biphasic effect of stimulating mouse hepatic microsomal parathion oxidation at a concentration of 1 microM, and inhibiting this same activity when increased to 100 microM. In contrast, 7,8-benzoflavone was only inhibitory at all concentrations examined. All the flavonoids examined except quercetin altered the ratio of activation/detoxification of parathion by mouse hepatic microsomes, but had no effect on this same ratio with human CYP1A2. These data suggest that the changes in the activation/detoxification ratio observed with mouse hepatic microsomes resulted from selective inhibition or stimulation of various cytochrome P450 isoforms rather than a flavonoid-induced alteration in the nonenzymatic rearrangement of the putative phosphooxythirane intermediate generated by cytochromes P450 from parathion. Surprisingly, however, none of the four flavonoids in the current study affected the lethality of parathion in vivo, suggesting that the flavonoid-induced alterations in cytochrome P-450-dependent metabolism of parathion documented in vitro were simply not great enough to be of any significance in vivo.