The effect of catechol on the development of urinary bladder tumors in Fischer rats treated with N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) was studied. A solution of 0.05% catechol and 0.001% BBN was administered in the drinking water ad libitum for 78 weeks. The urinary bladders were then removed and examined microscopically. No statistically significant difference was observed between the experimental group receiving 0.001% BBN with 0.05% catechol and animals receiving 0.001% BBN alone with respect to the incidence of hyperplasia, papilloma, or carcinoma of the urinary bladder. Animals receiving 0.05% catechol alone in the drinking water had no macroscopic or microscopic lesions significantly different from those in control animals receiving tap water. Analyses of the urine of animals receiving catechol in their drinking water indicated that greater than 99% of the catechol present was in the form of either glucuronide or sulfate conjugates. In a second bioassay, the potential cocarcinogenicity of catechol, administered together with BBN, was explored by direct instillation into the urinary bladder. The development of calculi and possible infections of the urinary tract within all groups of treated animals suggests that this bioassay technique for cocarcinogenicity is of questionable value. These data show that catechol at the dose and mode of administration employed in this study did not affect the epithelium of the urinary bladder or enhance the carcinogenic activity of BBN.