A dose-response study was carried out in male F344 rats with a specific urinary bladder carcinogen, N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN), which was administered as a solution in drinking water for up to 112 weeks. BBN was given in four different concentrations, 50, 10, 5, and 1 ppm, to groups of 30 rats. The highest dose induced urinary bladder cancer in all rats treated by week 91, decreasing yields being associated with lower dose levels: an incidence of 76.7% was observed with 10 ppm, followed by 20.0% with 5 ppm. The lowest dose (1 ppm) only induced two cases of papilloma (6.9%).