The effect of intravesical instillation of dimethylsulfoxide (DMSO) on bladder carcinogenesis was examined in mice. Experiment 1: Fifty-five female C3H/He mice were administered 0.05% N-butyl-N-(4-hydroxy-butyl) nitrosamine (BBN) in their drinking water for 8 weeks. In week 9 they were divided into two groups consisting of 25 mice each. Then, under nembutal anesthesia the first group was given weekly intravesical inatillations of 0.1 ml DMSO (minimum 99.0%) for 10 weeks. The second group received no treatment except anesthesia. All mice were killed 30 weeks after the begining of the experiment and their urinary bladder resected for histological examination. The incidence of bladder carcinoma was 93.7% (15.16) and 27.7% (6/22) in groups 1 and 2, respectively. These incidences differed significantly between the two groups. Experiment 2: One hundred and twenty female C3H/He mice were divided into two groups. The first group was given 0.05% BBN in their drinking water for 5 weeks and then tap water. The second group was not given BBN. In week 6, the first group was divided again into three groups (1, 2 and 3) consisting of 28, 26, and 27 mice, respectively. The second group was divided into groups 4 and 5 consisting of 21 and 18 mice, respectively. Under nembutar anaesthesia groups 1 and 4 received weekly intravesical instillation of 0.05 ml DMSO (minimum 99.0%) from weeks 6 to 13, Group 2 received weekly intravesical instillation of 0.05 ml distilled water from weeks 6 to 13. Groups 3 and 5 received no treatment except anesthesia.(ABSTRACT TRUNCATED AT 250 WORDS)