The effects of altering the hepatic mixed-function oxidase(MFO) activities on the acute toxicity of parathion were examined in female rats. Phenobarbital sodium pretreatment (50mg/kg/day, i.p.) for 4 consecutive days has resulted in significant decreases in the toxicity of parathion (2 or 4 mg/kg, i.p.) as determined by lethality and cholinesterase activities wheras the toxicity arising from a single dose of CCl4(2 mmol/kg, i.p.) 24 hr prior to parathion challenge was potentiated.