Background: Epidemiologic studies indicate that moderate ethanol consumption reduces cardiovascular mortality. Cellular and animal data suggest that ethanol confers beneficial effects on the vascular endothelium and increases the bioavailability of nitric oxide. Objectives: The purpose of this study was to assess the effect of ethanol on endothelium-dependent, nitric oxide-mediated vasodilation in healthy human subjects. Methods: Forearm blood flow, (FBF), was determined by venous occlusion plethysmography in healthy human subjects during intra-arterial infusions of either methacholine (0.3, 1.0, 3.0, and 10.0 mcg/min, n = 9), nitroprusside (0.3, 1.0, 3.0, and 10.0 mcg/min, n = 9), or verapamil (10, 30, 100, and 300 mcg/min, n = 8), before and during the concomitant intra-arterial infusions of ethanol (10% ethanol in 5% dextrose). Additionally, a time control experiment was conducted, duringwhich the methacholine dose response curve was measured twice during vehicle infusions, (n=5). Results: During ethanol infusion, mean forearm and systemic alcohol levels were 227±30 and 6± 0 mg/dL, respectively. Ethanol infusion alone reduced FBF (2.5±0.1 to 1.9±0.1 ml/dL/min, p<0.05). Despite initial vasoconstriction, ethanol augmented the FBF dose response curves to methacholine, nitroprusside, and verapamil (p <0.01 by ANOVA for each). To determine whether this augmented FBF response was related to shear-stress induced release of nitric oxide, FBF was measured during the co-infusion of ethanol and N-nitro-L-arginine methyl ester (L-NAME), (n=8), at rest and during verapamil-induced vasodilation. The addition of L-NAME did not block ethanol's ability to augment verapamil-induced vasodilation. Conclusions: Ethanol has complex direct vascular effects, which include basal vasoconstriction as well as potentiation of both endothelium-dependent and -independent vasodilation. None of these effects appear to be mediated by an increase in nitric oxide bioavailability, thus disputing findings from pre-clinical models.