The endothelium is a physical barrier between the blood and vascular smooth muscle, a source of enzymes activating and deactivating cardiovascular hormones and a site of production of relaxing and contracting factors. In addition, the endothelium is a source of growth inhibitors and promoters of vascular smooth muscle cells. Monoaminooxidase deactivates catecholamines and serotonin. Angiotensin converting enzyme transforms angiotensin I into angiotensin II and breaks down bradykinin into inactive products. Nitric oxide is a potent vasodilator and inhibitor of platelet function that under most circumstances is released together with prostacyclin, which exerts similar effects. Both substances play an important protective role in the coronary circulation in that they cause continuous vasodilation and inhibition of platelet function. In addition, the endothelium is a source of contracting factors such as endothelin-1, thromboxane A2, and endoperoxides. Endothelium-derived growth inhibitors include heparin (sulfates) and transforming growth factor beta 1, while basic fibroblast growth factors and platelet-derived growth factor and possibly endothelin promote proliferation. Because of its strategic anatomic position, the endothelium is a primary target for injuries and cardiovascular risk factors. In particular, aging, low density lipoproteins, hypertension, diabetes, and ischemia alter endothelium function. In arterial coronary bypass grafts, the release of nitric oxide is more pronounced than in vein grafts. Alterations of endothelial function may contribute to vasospasm, thrombus formation, and vascular proliferation and in turn myocardial ischemia, all common events in patients with coronary artery disease.