The in vitro effect of various concentrations of captan on hepatic microsomal cytochrome P-450 from pehnobarbital-pretreated rats was studied. The I-50 value, namely the concentration of the inhibitor necessary to produce 50% loss of cytochrome P-450 was determined from theplotted inhibition curve. The presence of ethylenediaminetetraacetic acid (EDTA) in microsomal incubations prior to the addition of captan failed to prevent the loss of cytochrome P-450 by captan. In contrast, reduced glutathione (0.5 mM) added to microsomal incubations before captan (0.1 mM) afforded almost complete protection of cytochrome P-450 from captan inhibition. These data indicate that the inhibitory effect of captan on vitally important drug-metabolizing enzyme system, of which cytochrome P-450 is a major component, can be prevented by prior presence of reduced glutathione (GSH) but not of EDTA.