Abstract Saccharin has been reported to induce urinary bladder tumors in multigeneration rat feeding studies and to promote bladder carcinogenesis in rats initiated with known bladder carcinogens. To examine the dose-dependent effects of saccharin on tumor promotion, sodium saccharin was administered at six levels in the diet (5.0, 2.5, 1.0, 0.5, 0.1, and 0%) to female Sprague-Dawley rats which had received, by trans-urethral instillation into the bladder, either a single dose of saline or an initiating dose of N-methyl- N-nitrosourea (MNU), a potent directacting carcinogen. Additional groups with and without MNU treatment received (1) sodium saccharin (2%) in the drinking water, (2) acid-saccharin (5%) in the diet, or (3) MNU, four weekly doses, as a positive control. Histopathologic examination of the urinary bladders from dead and moribound animals and from animals sacrificed after 102 weeks on dose was performed, and benign papillomas were commonly observed in those animals given MNU. A statistical analysis of the lesions indicated an increase in tumor incidence and a decrease in time to tumor with increasing saccharin dose in dead and moribund animals. This response was observed in the dose series of 0 to 2.5% saccharin in the diet. Dead and moribund animals which had received 5% sodium saccharin exhibited few tumors. An increasing incidence of tumors in MNU-treated control animals was observed during the final weeks of the study. Although this increase in background tumors in senescent animals complicated the interpretation of the total tumor incidences, the results in dead and moribund animals (about 60% of the total) indicated that saccharin served as a tumor promoter in this two-stage carcinogenesis model system by decreasing the latency period of the lesions.