We investigated whether different concentrations of the flavonoid quercetin ameliorate nitric oxide production and nuclear factor (NF)-kappaB activation in interleukin (IL)-1beta-activated rat hepatocytes. Primary cultures of rat hepatocytes were treated with IL-1beta alone or with quercetin in concentrations ranging from 5 to 100 micromol/L. The generation of reactive oxygen species, assessed by flow cytometry using dichlorodihydrofluorescein diacetate, was significantly reduced, and the oxidized:reduced glutathione ratio decreased in cultures treated with 50 and 100 micromol/L of quercetin. Quercetin at 100 micromol/L significantly prevented the IL-1beta-induced release of nitrite into the culture medium. Western blot and reverse transcription-PCR analyses demonstrated that increased levels of inducible nitric oxide synthase (iNOS) protein and mRNA in hepatocytes stimulated by IL-1beta were prevented by 50 micromol/L and 100 micromol/L of quercetin. Electrophoretic mobility shift assay experiments and Western blots indicated that quercetin blocked the activation of NF-kappaB and decreased the inhibitor kappaB protein levels induced by IL-1beta. In summary, quercetin, a natural flavonol widely distributed in the human diet, inhibits NO production in IL-1beta-stimulated hepatocytes through the inhibition of iNOS expression. Although the mode of action remains to be clarified, our findings support the view that the mechanism of action is via inhibition of IL-1beta-induced NF-kappaB activation.