Four compounds forming metabolic intermediate complexes with cytochrome P-450 in vitro were studied for their effects on hepatic glutathione in the mouse. All four compounds depleted glutathione within 1-3 hr after administration. The effect was transient for piperonyl butoxide while lasting at least 24 hr for other compounds. Induction of the mixed-function oxidase system by phenobarbital had no effect on the glutathione-depleting actions of the compounds, but induction with 3-methylcholanthrene abolished the depletion seen with piperonyl butoxide and SKF 525-A. For SKF 525-A, L-alpha-acetylmethadol (LAAM) and norLAAM, the persistent lowering of glutathione was paralleled by elevations in serum glutamic-pyruvic transaminase (SGPT) activity. This depletion of glutathione and subsequent elevations in SGPT were found to be strain and species dependent for SKF 525-A, LAAM and norLAAM. Compounds which complex with cytochrome P-450 in vitro may increase drug toxicities in vivo by mechanisms other than inhibition of oxidative drug metabolism.