We tested the effect of brief intraluminal perfusion of rat aortas with 5 mM hydrogen peroxide (H2O2) on basal and stimulated endothelium-dependent relaxation. Both 10min perfusion with H2O2and pharmacologic inhibition of endothelium-derived relaxing factor/ nitric oxide (EDRFINO) by 100μM Nω-nitro-L-arginine increased the sensitivity to the vasoconstrictor phenylephrine 10-fold, consistent with a reduction in basal EDRF/NO. Paradoxically, 10 min perfusion with H2O2enhanced sensitivity to both the endothelium-dependent dilator acetylcholine (3-fold reduction in EC50), and the endothLelium-independent dilator sodium nitroprusside (5-fold reduction in EC50), while more prolonged exposure to H2O2selectively reduced endothelium-dependent relaxation. These resullts suggest differential sensitivity of basal and stimulated EDRF/NO to oxidative stress, and further that inhibition of basal EDRF/NO by H2O2enhances the activity of both endothelium-dependent and -independent vasodilators, consistent with functional antagonism between basal and stimulated EDRF/NO.