Sodium saccharin (NaSac) fed as 5% of Prolab diet promotes bladder tumor carcinogenesis in male F344 rats initiated with N-[4-(5-nitro-2-furyl)-2-thiazolyl]formamide (FANFT) fed as 0.2% of the diet for 4 weeks. NaSac also increases urothelial proliferation if fed for short periods in Prolab diet, but no increased proliferation is seen if it is fed for up to 10 weeks in AIN-76A semisynthetic diet, even at levels as high as 7.5% of the diet. To determine whether NaSac as part of an AIN-76A diet has promoting activity, groups of approximately 30 male, 5-week-old F344 rats were fed AIN-76A diet containing (a) 0.2% FANFT for 4 weeks followed by 5% NaSac for 100 weeks; (b) 0.2% FANFT followed by control diet; or (c) control diet followed by 5% NaSac. Bladder tumor incidences were 10, 23, and 0%, respectively. When fed in Prolab diet, 0.2% FANFT for 4 weeks followed by NaSac or control diet for 100 weeks resulted in bladder tumor incidences of 40 and 17%, respectively. Groups of rats fed 0.1 or 0.2% FANFT for 1 or 2 weeks followed by 5% NaSac or control diet for 100 weeks had bladder tumor incidences of 0 to 7%. These data demonstrate that NaSac does not promote bladder cancer in male rats if fed in AIN-76A diet. Other studies suggest that this is due to the low urinary pH in rats fed AIN-76A diet.