Primary rat bladder epithelial cells were coculivated with Chinese hamster V79 cells in the presence of carcinogens, and the induction of 6-thioguanine resistance in the V79 cells was used as a marker of cell-mediated mutagenesis. The carcinogens dimethylnitrosamine, 7, 12-dimethylbenz(a)anthracene, and benzo(a)pyrene (BP) were mutagenic to V79 cells in the presence of bladder cells but not in their absence. Analysis of BP metabolites formed by bladder cells indicated that 7,8-dihydro-7,8-dihydroxybenzo(a)pyrene, 9,10-dihydro-9,10-dihydroxybenzo(a)pyrene, benzo(a)pyrene-3,6-quinone, and 9-hydroxybenzo(a)pyrene were the major organic-soluble metabolites formed. Glucuronide and sulfate conjugates of BP metabolites were also produced by bladder cells. Mutagenesis data from the rat bladder system and previous data from rat liver and lung cell-mediated mutagenesis systems indicate that the cell-mediated mutagenesis approach may provide a useful approach for studying the organotropic effect of chemical carcinogens. Furthermore the finding that rat bladder epithelium can metabolize some carcinogens offers new possibilities for the mechanism of initiation of bladder cancer.