Arylhydrocarbon receptor knock-out, AhR(–/–), mice have recently been shown to be rather resistant to benzo[ a ]pyrene (B[ a ]P)-induced tumor formation, probably reflecting the inability of these mice to express significant levels of cytochrome P450 (P450 or CYP) 1A1 that activates B[ a ]P to reactive metabolites (Y.Shimizu, Y.Nakatsuru, M.Ichinose, Y.Takahashi, H.Kume, J.Mimura, Y.Fujii-Kuriyama and T.Ishikawa (2000) Proc. Natl Acad. Sci. USA , 97, 779–782). However, it is not precisely determined whether CYP1B1, another enzyme that is also active in activating B[ a ]P, plays a role in the B[ a ]P carcinogenesis in mice. To understand the basis of roles of CYP1A1 and CYP1B1 in the activation of chemical carcinogens, we compared levels of induction of liver and lung CYP1A1, 1A2, and 1B1 by various polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls in AhR(+/+) and AhR(–/–) mice. Liver and lung CYP1A1 and 1B1 mRNAs were highly induced in AhR(+/+) mice by a single intraperitoneal injection of each of the carcinogenic PAHs, such as B[ a ]P, 7,12-dimethylbenz[ a ]anthracene, dibenz[ a,l ]pyrene, 3-methylcholanthrene, 1,2,5,6-dibenzanthracene, benzo[ b ]fluoranthene, and benzo[ a ]anthracene and by a co-planar PCB congener 3,4,3′,4′-tetrachlorobiphenyl. We also found that 6-aminochrysene, chrysene, benzo[ e ]pyrene, and 1-nitropyrene weakly induced the mRNA expression of CYP1A1 and 1B1, whereas anthracene, pyrene, and fluoranthene that have been reported to be non-carcinogenic in rodents, were very low or inactive in inducing these P450s. The extents of induction of liver CYP1A2 by these chemicals were less than those of CYP1A1 and 1B1 in AhR(±/±) mice. In AhR(–/–) mice, there was no induction of these P450s by PAHs and polychlorinated biphenyls. Liver microsomal activities of 7-ethoxyresorufin and 7-ethoxycoumarin O -deethylations and of mutagenic activation of (±)- trans -7,8-dihydroxy-7,8-dihydro-B[ a ]P to DNA-damaging products were found to correlate with levels of CYP1A1 and 1B1 mRNAs in the liver. Our results suggest that carcinogenicity potencies of PAHs may relate to the potencies of these compounds to induce CYP1A1 and 1B1 through AhR-dependent manner and that these induced P450s participate in the activation of B[ a ]P and related carcinogens causing initiation of cancers in mice.