The endothelial cells lining the luminal surface of blood vessels are involved in numerous regulatory functions, such as control of vascular smooth muscle cell proliferation, adhesion of leukocytes and platelets, permeability, and inflammatory responses. The endothelium also has thrombolytic and fibrinolytic properties. Its metabolic activity contributes to the regulation of the oxidation of plasma lipids, angiotensin II formation, and the degradation of circulating catecholamines and kinins. In addition, the endothelium plays an important role in the regulation of vascular smooth muscle tone by releasing both relaxing and contracting factors. Endothelium-dependent relaxations are mediated primarily by nitric oxide, but also by endotheliumderived hyperpolarizing factor (EDHF) and prostacyclin. The contracting factors are endothelin-1, metabolites from the cyclooxygenase pathway, and superoxide anions. Under physiological conditions, a precise and balanced release of relaxing and contracting factors ensures adequate organ perfusion. However, this balance is altered in disease states such as atherosclerosis, diabetes, chronic heart failure, coronary artery disease, or hypertension, thereby contributing to the further development of vascular diseases...