Quercetin is a naturally occurring chemical found in our daily diet in fruits and vegetables. Toxicity and carcinogenicity studies of quercetin were conducted in male and female F344/N rats, under conditions which allowed comparison to results of approximately 400 previously tested chemicals. The chemical was administered in the feed for 2-years at concentrations of 0, 1000, 10,000, or 40,000 ppm, and the estimated dose delivered was approximately 40-1900 mg/kg/day. There were no treatment-related effects on survival and no treatment-related clinical signs of toxicity. The high-dose groups had reduced body weight gain in comparison to controls during the last half of the study. At interim evaluations at 6 and 15 months, treatment-related toxic lesions were not observed, but at 2 years toxic and neoplastic lesions were seen in the kidney of male rats, including increased severity of chronic nephropathy, hyperplasia, and neoplasia of the renal tubular epithelium. Under the conditions of these 2-year studies quercetin showed carcinogenic activity in the kidney of the male rat, causing primarily benign tumors of the renal tubular epithelium. Quercetin did not cause tumors at other sites. Quercetin is a genotoxic chemical, but the neoplastic response observed in the kidney may be due in part to a combination of nongenotoxic and genotoxic events.