Objectives: i To compare the basal expression of the Ugt1a gene in metabolically important mouse tissues. ii To determine the change in the expression of the Ugt1a gene in the liver, kidney, and small intestine after acute treatment with BBN. iii To determine whether the reduction of microbiota with antibiotics affects the expression of the Ugt system in the urinary bladder of mice treated with BBN. iv To determine whether there is an altered expression of individual isoforms of the Ugt genes in mice bladder tumors. Materials and methods: The experiment was conducted in the Cancer Research Laboratory at the Department of Immunology and Medical Genetics, University of Split School of Medicine. Of the seven C57BL/6J mice, four took BBN in drinking water (experimental group), and three drank water (control group). The expression of the Ugt1a gene in the liver, kidney, duodenum, jejunum, and femoral muscle was quantified using the qRT-PCR method. Furthermore, the activity of the glucuronidation system genes was analyzed using previously collected RNA sequencing data of urinary bladder tumor tissue as well as mouse urinary bladder tissue after antibiotic reduction of microbiota and two-week BBN treatment. Results: Liver, duodenum, jejunum, and kidney have significantly higher expression of the main glucuronidation gene Ugt1a compared to the skeletal muscle. Compared to the liver, only the kidney showed a significantly higher expression of the Ugt1a gene. No significant change in Ugt1a gene expression was found after treatment of mice with BBN. In the murine urinary bladder tumor tissue, a significantly lower expression of six Ugt isoforms was observed, namely: Ugt1a6a, Ugt1a6b, Ugt1a7c, Ugt2b1, Ugt2b34 and Ugt2b35. A statistically significant lower expression of the Ugt1a7c gene was observed in the urinary bladder of mice with antibiotic-reduced microbiota and treated with BBN. Conclusion: The results of this study indicate that the liver is not the only organ with significant glucuronidation capacity, as a high level of Ugt1a gene expression was found in the kidney and small intestine. BBN consumption did not significantly change the expression of the Ugt1a gene in metabolically relevant mouse tissues. The microbiota could also play an important role in the regulation of urinary bladder Ugt genes expression. Decreased expression of genes for enzymes of the glucuronidation system in urinary bladder tumor tissue indicates that a possible decrease in glucuronidation capacity may be related to the urothelial carcinogenesis.