Resveratrol, naringenin and naringin are naturally occurring flavonoids in grapes and grapefruits. The anti-inflammatory effects of these flavonoids have been well documented, but the mechanism is poorly characterized. High concentration of NO are produced by inducible NO synthase (iNOS) in inflammation, and the prevention of the expression of iNOS may be an important anti-inflammatory mechanism. In this study, the effects of these flavonoids on the induction of NO synthase (NOS) in RAW 264.7 cells activated with bacterial lipopolysaccharide (LPS, 50 ng ml(-1)) were investigated. Resveratrol was found strongly to inhibit NO generation in activated macrophages, as measured by the amount of nitrite released into the culture medium, and resveratrol strongly reduced the amount of cytosolic iNOS protein and steady state mRNA levels. However, the inhibitory abilities of naringenin were lower, and the inhibitory abilities of naringin were almost negligible. In electrophoretic mobility shift assays, the activation of NFkappaB induced by LPS for 1 h was inhibited by resveratrol (30 microM). Furthermore, in immunoblotting analysis, cells treated with LPS plus resveratrol showed an inhibition of phosphorylation as well as degradation of IkappaBalpha, and a reduced nuclear content of NFkappaB subunits. The flavonoids may be of value for inhibiting the enhanced expression of iNOS in inflammation through down-regulation of NFkappaB binding activity.