The vascular endothelium was long regarded as simply a diffusion barrier, but now it is recognized that endothelium is a metabolically active mediator of interactions between blood and underlying smooth muscle. Metabolic functions of endothelial cells include the enzymatic destruction of serotonin, norepinephrine, the conversion of angiotensin I to potent vasoconstrictor angiotensin II and the breakdown of potent vasodilator bradyki- nin. Endothelial cells also synthesize and secrete vasodilator and antiaggregant prostacyclin and endothelium derived relaxing factor. These cells are normally non- thrombogenic and unstimulated platelets do not adhere to the surface but when the cells are injured platelets adhere to the exposed subendothelium. Coagulation can be initiated by damaged endothelial cells because they bind coagulation factors and may activate them. Endothelium can control coagulation by activation of anticoagulant protein C and by increasing the affinity of antithrombin III for thrombin. Endothelial cells contribute to the process of fibrinolysis by synthesizing plasminogen activators and plasminogen activator inhibitors.