Metabolism of polychlorinated dibenzo-p-dioxins by CYP1A subfamily was examined by using the recombinant yeast microsomes. In substrate specificity and reaction specificity, considerable species differences between rats and humans were observed in both CYP1A1- and CYP1A2-dependent metabolism of dioxins. Among four CYPs, rat CYP1A1 showed the highest activity toward dibenzo-p-dioxin (DD) and mono-, di-, and trichloroDDs. To reveal the mechanism of dioxin metabolism, we examined rat CYP1A1-dependent metabolism of 2-chloro-dibenzo-p-dioxin. In addition to hydroxylation at an unsubstituted position, hydroxylation with migration of a chloride substituent, hydroxylation with elimination of a chloride substituent, and cleavage of an ether linkage of the dioxin ring were observed. In particular, the cleavage of an ether linkage of the dioxin ring appeared most important for the detoxication of dioxins. Based on these results, the metabolic pathways of 2-chloro-dibenzo-p-dioxin by rat CYP1A1 were proposed. The metabolic pathways contain most of the metabolites observed in vivo using experimental animals, suggesting that P450 monooxygenase systems including CYP1A1 are greatly responsible for dioxin metabolism in vivo.