Rats were treated, during 8 weeks, with different doses of edifenphos (0, 5, 10, 20, 40 mg/kg/day) incorporated in the food. Treatments with 20 and 40 mg/kg increased the relative liver weight and hepatic concentrations of phospholipids, microsomal proteins and cytochrome P450. Aminopyrine N-demethylase and aniline hydroxylase activities were increased at the same doses. Hepatic lipids and blood triacylglycerols remained unchanged by the treatment, but blood cholesterol increased significantly (33%) at 40 mg/kg. Plasma cholinesterase activity was slightly depressed in rats treated with the two higher doses. Under these experimental conditions, the dose of edifenphos, inducing no significant difference was 10 mg/kg/day.