The goal of the present study was to evaluate fipronil effects on the activities of drug metabolizing enzymes in rat liver microsomes. Rats were orally treated with fipronil at doses of 1, 5, 10 and 15 mg/kg bw/day for 6 days. Determinations of cytochrome P450 (CYP) enzyme activities were carried out in hepatic microsomes isolated from treated rats. The activities of some members of CYP2E, CYP1A, CYP2A, CYP2B and CYP3A subfamilies significantly increased after fipronil treatment in a dose-dependent manner as compared to control. The major effects were observed in the O-deethylation of ethoxyresorufin and O-demethylation of methoxyresorufin (reflecting CYP1A1/2 activities), in the O-depenthylation of pentoxyresorufin and 16β-hydroxylation of testosterone (reflecting CYP2B1/2 activities), and in the N-demethylation of erythromycin and 6β-hydroxylation of testosterone (reflecting CYP3A1/2 activities). Immunoblot studies revealed that fipronil increased the apoprotein levels of CYP1A1. Our results suggest that fipronil is an inducer of hepatic phase I CYP enzymes, causing an increased potential to interact with a wide range of xenobiotics or endogenous chemicals that are substrates of the CYP1A, CYP2B and CYP3A subfamilies. Further investigations are required to in vivo evaluate the potential of the metabolite fipronil sulfone as an inducer of phase I CYP enzymes.