1. Both resveratrol and quercetin dose-dependently inhibited the contractile response to noradrenaline (NA) in isolated endothelium-intact rat aorta. This inhibitory effect on vascular contraction was blocked by pretreatment of the blood vessel with the nitric oxide (NO) synthase inhibitor, L-NNA (1 microM). 2. Quercetin at a concentration > 1 x 10(-5) M, and resveratrol at > 3 x 10(-5) M, caused relaxation of the phenylephrine (PE) precontracted endothelium-intact aorta, and L-NNA, at 1 x 10(-6) M, reversed the relaxation. 3. At higher concentrations, > 6 x 10(-5) M, resveratrol and quercetin also relaxed the endothelium-denuded aortic rings. However, this effect could not be reversed by the NO inhibitor. 4. It is concluded that resveratrol and quercetin exert both indirect and direct vasodilator effects on the blood vessel by nitric oxide-mediated and non-NO-mediated mechanisms, respectively.