2-Amino-1-methyl-6-phenylimidazo[4,5- b ]pyridine (PhIP) carcinogenesis is initiated by N² -hydroxylation, mediated by several cytochromes P450, including CYP1A1. However, the role of CYP1A1 in PhIP metabolic activation in vivo is unclear. In this study, Cyp1a1 -null and wild-type (WT) mice were used to investigate the potential role of CYP1A1 in PhIP metabolic activation in vivo . PhIP N² -hydroxylation was actively catalyzed by lung homogenates of WT mice, at a rate of 14.9 ± 5.0 pmol/min/g tissue, but <1 pmol/min/g tissue in stomach and small intestine, and almost undetectable in mammary gland and colon. PhIP N² -hydroxylation catalyzed by lung homogenates of Cyp1a1 -null mice was ∼10-fold lower than that of WT mice. In contrast, PhIP N² -hydroxylation activity in lung homogenates of Cyp1a2 -null versus WT mice was not decreased. Pretreatment with 2,3,7,8-tetrachlorodibenzo- p -dioxin increased lung Cyp1a1 mRNA and lung homogenate PhIP N² -hydroxylase activity ∼50-fold in WT mice, where the activity was substantially inhibited (70%) by monoclonal antibodies against CYP1A1. In vivo , 30 min after oral treatment with PhIP, PhIP levels in lung were similar to those in liver. After a single dose of 0.1 mg/kg [ ¹⁴ C]PhIP, lung PhIP-DNA adduct levels in Cyp1a1 -null mice, but not in Cyp1a2 -null mice, were significantly lower ( P = 0.0028) than in WT mice. These results reveal that mouse lung has basal and inducible PhIP N² -hydroxylase activity predominantly catalyzed by CYP1A1. Because of the high inducibility of human CYP1A1, especially in cigarette smokers, the role of lung CYP1A1 in PhIP carcinogenesis should be considered. (237 words).