The synthetic pyrethroid, permethrin, was evaluated for its ability to alter hepatic microsomal drug-metabolizing function. The influence of permethrin (25:75 cis-trans) on plasma antipyrine kinetics and gamma-glutamyl transpeptidase (gamma-GTP) activity were studied in rats. After 3 days of administration of 90 mg permethrin/kg/day, there was no significant change in the antipyrine half-life and the area under the curve, while the apparent volume of distribution and clearance were significantly increased. Treatment with 190 mg permethrin/kg/day for 3 days decreased antipyrine half-life and the area under the curve, and increased the apparent volume of distribution and the clearance significantly. The gamma-GTP activity was significantly increased within 21 days and 14 days after the start of permethrin administration, at doses of 90 and 190 mg permethrin/kg/day, respectively. The antipyrine kinetics results indicate that permethrin is capable of producing a dose-dependent marked enzyme-inducing effect.