We have investigated the regulation and expression of CYP1A1 in solid and papillary lung tumors induced by the carcinogen urethane. Female strain A/J mice were administered urethane (1 mg/g body wt) intraperitoneally, and when lung tumors were established at 16 weeks, mice were treated with 3-methylcholanthrene to induce CYP1A1. CYP1A1 mRNA was detected by in situ hybridization with a 3H-labeled RNA probe and quantitated by image analysis, whereas CYP1A1 protein was detected by immunohistochemical staining with an avidin-biotin complex procedure. Our results showed that in untreated control and tumor-bearing mice, the CYP1A1 mRNA was present at low levels, but the CYP1A1 protein was not detectable. Treatment with 3-methylcholanthrene induced increased levels of both CYP1A1 mRNA and protein in lung parenchyma and tumor foci. This induction was markedly higher in the parenchyma of control and tumor-bearing lungs than in either solid or papillary tumors. Differences in CYP1A1 mRNA and protein expression were not evident in solid and papillary tumors. In the parenchyma, induced CYP1A1 mRNA and protein were localized in the endothelial and alveolar septal cells. Endothelial cells in tumors also contained substantial CYP1A1 mRNA and protein, whereas low levels of both were found in lung tumor cells. Our finding of significant reduction in inducibility of CYP1A1 protein in conjunction with reduced CYP1A1 mRNA in tumor foci suggests reduced transcriptional activation as a regulatory mechanism. The lack of association between diminished CYP1A1 expression and either the tumor type or the mechanism mediating metabolic activation supports the hypothesis that the persistence of lung tumors may be due, in part, to a restricted capability for the formation of reactive intermediates.