Nitric oxide is the active chemical species responsible for the vasodilator action of nitroglycerin, nitroprusside, and related nitrovasodilators. The most potent vasodilator and inhibitor of platelet aggregation known, nitric oxide was recently discovered to occur endogenously as the endothelium-derived relaxing factor. The pharmacology of endothelium-derived nitric oxide is virtually identical to that of the clinically used nitrovasodilators. Although endothelium-derived relaxing factor or endothelium-derived nitric oxide seems to be important in animals, its significance in humans still needs to be shown. We review the recent discoveries in the identification, biosynthesis, metabolism, and biologic actions of endothelium-derived nitric oxide, its significance in humans, and its relation to the clinically used nitrovasodilators.