The distribution of endothelin-1 (ET-1) receptors on human vascular tissue has been studied. High- and low-resolution autoradiography was used to determine the distribution of [125]ET binding sites in human blood vessels and ventricular myocardium. Dense, displaceable [125I]ET binding was associated with cardiac myocytes and the smooth muscle layer of all vessels were examined. There was also dense binding to vasa vasora. There was increased [125I]ET binding to atheromatous coronary arteries and vein graft, which was associated with the tunica media and vasa vasora or regions of neovascularization. Vasoconstrictor and positive inotropic activity of ET-1 has been established in vitro. The vasoconstrictor effect of ET-1 is likely to be mediated via the binding sites identified on vascular smooth muscle. The striking perivascular [125I]ET-1 binding suggests that ET-1 may also have constrictor activity on vasa vasora. There is experimental evidence that ET-1 has mitogenic activity on vascular smooth muscle cells in culture. The increased binding to both smooth muscle and regions of neovascularization in atheromatous vessels suggests that ET-1 may play a role in atherosclerosis.