Haloperidol (HP), an antipsychotic drug, is N-dealkylated by cytochrome P450 (CYP) to 4-fluorobenzoylpropionic acid (FBPA). The purpose of this study was to identify whether CYP3A metabolizes HP to FBPA in hepatic microsomes of rats and to investigate whether an inhibitor or an inducer of CYP3A affects HP pharmacokinetics in rats. The rate of FBPA formation was determined in hepatic microsomes from 8-week-old male Sprague-Dawley rats. Among several specific CYP isozyme inhibitors including troleandomycin (TAO), diethyldithiocarbamate, furafylline and quinine, only TAO showed marked inhibition of FBPA formation. Anti-rat CYP3A serum inhibited FBPA formation by 76.4%, while other anti-rat CYP sera (1A1, 1A2, 2B1, 2C11, 2E1) only slightly did. In a pharmacokinetic study, 8-week-old male Sprague-Dawley rats were given 0.5 mg/kg HP intravenously after treatment with 100 mg/kg erythromycin, a CYP3A inhibitor, or 80 mg/kg dexamethasone, a CYP3A inducer, intraperitoneally once a day for 7 days or 2 days, respectively or untreated. HP half-life was prolongated to 171% of the average control value by erythromycin and shortened to 49% of control by dexamethasone. HP clearance was reduced to 63% of control by erythromycin and was increased to 167% of control by dexamethasone. These results suggested that CYP3A mainly catalyzed HP to FBPA in rats, and the modification of this enzyme activity would affect the pharmacokinetics of HP.