Paraquat inhibits thein vitrohepatic microsomal metabolism of both ethylmorphine and aniline. Inclusion of ascorbate with paraquat in the incubations did not alter the paraquat effect on ethylmorphine N‐demethylase activity but potentiated the inhibition of aniline p‐hydroxylase activity. Ascorbate alone was without effect on the metabolism of either substrate. Paraquat stimulated the hepatic microsomal oxidation of nicotinamide adenine dinucleotide phosphate (NADPH) equally in the absence of mixed‐function oxidase (MFO) substrates or in the presence of ethylmorphine; in the presence of aniline the rate of NADPH oxidation was significantly greater. Also, in the presence of aniline, ascorbate potentiated the paraquat‐induced NADPH oxidation, while it was ineffective with paraquat on NADPH oxidation in the presence of ethylmorphine or in the absence of substrates for the microsomal MFO system. The potentiated inhibition of aniline metabolism, concomitant with the potentiated stimulation of NADPH oxidation, was consistent whether liver microsomal fractions were prepared from control rats or from animals induced with phenobarbital. Investigation of possible influences on NADPH cytochrome c reductase activity was precluded by the rapid nonenzymatic reduction of cytochrome c by ascorbate. The paraquat‐ascorbate redox couple would not reduce cytochrome P‐450. These data suggest that a paraquat interaction with the active microsomal MFO enzyme system plays a role in the depletion of cellular NADPH stores that occurs after paraquat administrationin vivo. This mechanism may play a significant role in the development of paraquat toxicity and in the potentiated toxicity observed with ascorbate and paraquat.