Inhibition kinetics of cytochrome P-450 (P-450)-dependent monooxgenases by an environmental carcinogen safrole (1-allyl-3,4-methylenedioxybenzene) was studied in rat liver microsomes. The activities of P-450ⅠA1-specific ethoxyresorufin O-deethylase (EROD), P-450ⅡB-specific pentoxyresorufin O-dealkylase (PROD), and P-450ⅡE1-specific p-nitrophenol hydroxylase (PNPH) were inhibited dose-dependently when safrole was added into the reaction mixtures from 50 to 400 μM. Meanwhile, the P-450ⅢA1-specific erythromycin N-demethylase (ERDM) was not inhibited at all by safrole up to 400 μM. The inhibition pattern studies for each reaction in Dixon and Cornish-Bowden plots showed three distinct kinetics; mixed inhibition for EROD, uncompetitive for PROD, and noncompetitive for PNPH. The K_i` values of safrole for EROD and PNPH were 95.4 and 215.1μM respectively. The K_i` values of safrole for PROD was 9.48 μM. These results suggest that safrole inhibits microsomal P-450 isozyme-specific monooxygenases with distinct mechanisms and safrole could be used as an useful inhibitor to study P-450-mediated monooxygenase reactions.