The vascular endothelium is now widely recognized as an organ with a variety of functions important in the regulation of blood vessel tone and permeability, the coagulation of blood, the activity of leukocytes, and the reactivity of platelets. The endothelium contributes to the regulation of vascular tone by releasing powerful vasodilators such as prostacyclin [l] and endothelium-derived relaxing factor (EDRF;[2]) and by metabolizing vasoactive substances such as catecholamines, angiotensin, bradykinin and prostaglandins [3].

The role of the vascular endothelium as a modulator of platelet and leukocyte interactions with the vessel wall has been the subject of much attention, first as a result of the discovery that prostacyclin is an inhibitor of platelet aggregation and leukocyte activation [4] and, second, as a consequence of the finding that EDRF is not only an inhibitor of platelet aggregation [5] but also of platelet adhesion [6, 71. In this review we will discuss the evidence for the identification of EDRF as nitric oxide (NO) and for its role as a regulator of vascular tone and plateletvessel wall interactions.