The vascular endothelium synthesizes at least three potent vasodilator substances: nitric oxide (NO), prostacyclin and an endothelium-derived hyperpolarizing factor (EDHF). EDHF release is stimulated by receptor-dependent agonists such as acetylcholine and bradykinin (BK), and leads to hyperpolarization of the underlying smooth muscle cells presumably by opening Ca(2+)-activated K(+) channels. Indirect pharmacologic evidence suggests that EDHF is a cytochrome P450-derived arachidonic acid metabolite, presumably an epoxide.