Down-regulation of carcinogen detoxifying enzymes might be a critical factor in tumour formation by increasing the carcinogen concentration in the target organ. Previous reports revealed that the expression of UGT1A mRNA is either lost or decreased in certain human cancer tissues, including urinary bladder cancer. To elucidate this down-regulation mechanism, we used an N-nitrosobutyl (4-hydroxybutyl) amine (BBN)-induced mouse urinary bladder carcinogenesis model. Similar to human cancer, the expressions of Ugt1a6, Ugt1a9 and total Ugt1a mRNA in the BBN-induced bladder cancer were markedly decreased compared with those of normal mice. BBN down-regulated the basal Ugt1a mRNA expression in a time-dependent manner and this was reversible in the first 2 weeks of BBN treatment. However, after 4 weeks of BBN treatment the repression became persistent after the cessation of BBN treatment. Aryl hydrocarbon receptor (AhR) regulates the constitutive and inducible expression of Ugt1a mRNA. We found that the constitutive Ugt1a mRNA expression is decreased in the bladder of AhR knockout (KO) mice. Furthermore, BBN-induced Ugt1a down-regulation was lost in AhR KO mice, and the canonical AhR target gene Cyp1a1 was similarly down-regulated by BBN in the bladder. These results demonstrate that BBN repressed Ugt1a mRNA expression via suppression of AhR signaling pathway during BBN-induced carcinogenesis.