The human CYP1A1 gene is regulated by the aryl hydrocarbon receptor (AhR), and induction of CYP1A1 is known to play an important role in xenobiotic metabolism. To examine the regulation of human CYP1A1 in vivo, we created a transgenic mouse strain (Tg-CYP1A1(GFP)) expressing a chimeric gene consisting of the entire human CYP1A1 gene (15 kb) fused with a GFP reporter gene. The treatment of Tg-CYP1A1(GFP) mice with a single intraperitoneal dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or benzo[a]pyrene (B[a]P) led to the induction of CYP1A1(GFP) in both the liver and the lung as determined by fluorescence and Western blot analysis. The localization of induced fluorescence in liver also demonstrated the usefulness of cultured hepatocytes in examining the actions of AhR agonists toward induction of CYP1A1(GFP). Other routes of B[a]P administration, such as by oral exposure at 100 mg/kg for 3 days, led to reduced induction of CYP1A1(GFP) in liver and lung. In liver, expression of CYP1A1(GFP) was a sensitive marker for oral exposure, while mouse CYP1A1 was not induced at these doses. While first pass metabolism of B[a]P in the gastrointestinal tract reduces the potential of the AhR to induce CYP1A1(GFP) in the liver, adequate concentrations reach the hepatic circulation as demonstrated by induction of human UGT1A proteins in transgenic mice that express the human UGT1 locus. The capability to identify fluorescently labeled CYP1A1 in vivo provides a sensitive measurement of gene response and links exposure to potential environmental toxicants and activation of the AhR.