Effects of altering hepatic mixed-function oxidase (MFO) enzyme activities on the metabolism and acute toxicity of parathio were investigated in adult female rats. In vitro hepatic metabolism of parathion to paraoxon was increased by phenobarbital pretreatment (50 mg/kg/day, ip, for 4 consecutive days) and SKF 525-A (50 mg/kg, ip, 1 hr prior to sacrifice) decreased paraoxon formation indicating that phenobarbital induces that form(s) of cytochrome P-450 catalyzing conversion of parathion to paraoxon. Degradation of paraoxon to p-nitrophenol was increased by phenobarbital pretreatment, but not affected by SKF 525-A suggesting that MFO activities play only a minor role in the detoxification of the active metabolite of this insecticide. The phenobarbital-induced increase in paraoxon formation was partially antagonized by SKF 525-A. Significant activity for both parathion activation and paraoxon degradation was also observed in the lung preparation, however, this extrahepatic parathion and paraoxon metabolizing activity was not induced by phenobarbital or inhibited by SKF 525-A pretreatment. Phenobarbital pretreatment increased paraoxon level in livers of rats when measured 3 hr following parathion injection (2 mg/kg, ip). SKF 525-A did not alter parathion or paraoxon levels in brain, blood and liver. Phenobarbital pretreatment decreased the toxicity of parathion (4mg/kg, ip) or paraoxon (1.5 mg/kg, ip) as determined by decreases in lethality and inhibition of brain and lung acetylcholinesterases. An additional SKF 525-A treatment failed to decrease the protective effects of phenobarbital against parathion or paraoxon toxicity. These results suggest that some unknown factors other than hepatic MFO induction are involved in the protective action of phenobarbital against parathion and paraoxon toxicity.