Quercetin and kaempferol were tested for carcinogenicity in rats of both sexes. In Experiment I, 0.1% quercetin or control diet was given to Fischer 344 rats for 540 days. In experiment II, 0.04% kaempferol or control diet was given to ACI rats for 540 days. In both experiments, most tumors found in the experimental groups were also found in the corresponding control groups, and there were no statistical differences in the incidences of tumors in the experimental groups and the respective control groups. Quercetin and kaempferol were not shown to be carcinogenic to rats under these conditions.