Quercetin was administered at dietary levels of 0(control), 1.25 and 5.0% to groups of 50 male and 50 female rats for 104 weeks, and then all animals were maintained without quercetin supplement for a further 8 weeks. At 5.0% quercetin, both sexes showed growth retardation throughout the study. There were no treatment-ascribed effects regarding clinical signs, mortality, urinalyses or hematology. Although serum glucose in 5.0% quercetin-treated males was significantly decreased and some relative organ weights in 5.0% groups showed statistically significant increases, these latter changes seemed to be related to the growth retardation. An increased incidence of non-neoplastic hyperplastic polyps in the cecum was noted in the 5.0% males. The incidences of cystic changes and fibroadenomas of the mammary gland, and foci (areas) of hepatocellular alteration in the 5.0% females, and liver bile duct proliferations in the 5.0% males were significantly decreased. No proliferative lesions of the urinary bladder related to treatment with quercetin were found in any rats. The incidences of several other nonneoplastic and neoplastic lesions which demonstrated statistically significant changes appeared to be related to the growth retardation or to be within the normal range, and therefore none was considered to be significant biologically. Thus, the investigation did not demonstrate any clear carcinogenic effect of quercetin on F344 rats at dietary levels of up to 5.0%.