Sulfosulfuron, developed as a herbicide, caused increased microcrystalluria and the formation of urinary tract calculi when fed to male and female rats in a chronic 2-year study at doses of 5,000 ppm and 20,000 ppm. Hyperplasia was also seen in urinary bladders at 5,000 ppm and 20,000 ppm, almost exclusively in the presence of observable calculi/microcalculi. Urinary bladder tumors were found in 2 females in the 5000 ppm group, both in the presence of calculi. No increased microcrystalluria, calculi, or tumors were found at doses of 500 ppm and lower. In the current study, 5 groups of male Sprague-Dawley rats were fed sulfosulfuron at doses of 50, 500, 5,000, and 20,000 ppm for 10 weeks. Ten animals were co-administered 5,000 ppm sulfosulfuron with 12,300 ppm NH4Cl to determine if inhibition of the formation of calculi would prevent any urothelial effects of treatment with sulfosulfuron. Ten animals in the control group and in the high-dose sulfosulfuron group were fed only basal diet for an additional 10 weeks to determine if the effects of sulfosulfuron on the bladder epithelium were reversible. There was an increased incidence of microcrystalluria observed at 5,000 and 20,000 ppm. There was no increase in microcrystalluria observed in the urine of rats co-administered sulfosulfuron and NH4Cl. Urinary bladder calculi were found in the bladder of 1 animal fed 20,000 ppm. Examination by light microscopy showed diffuse papillary/nodular hyperplasia of the bladder epithelium in this animal. No increased microcrystalluria was observed after withdrawal of the chemical from the diet and the bladder epithelium was normal by light microscopy. The hyperplastic effects associated with the feeding of high doses of sulfosulfuron occur only with the appearance of urinary tract calculi. Based on these results and anatomical differences between rats and humans, it may be concluded that the hyperplastic and carcinogenic effects of sulfosulfuron in rats are high-dose, threshold phenomena that are not likely to occur in humans under environmentally relevant exposures.