There is abundant evidence that the endothelium plays a crucial role in the maintenance of vascular tone and structure. One of the major endothelium-derived vasoactive mediators is nitric oxide (NO), which has been characterized as an "endogenous anti-atherosclerotic molecule". Synthesis of NO can be selectively inhibited by guanidino-substituted analogs of L-arginine, which act as competitive inhibitors at the active site of the enzyme. One such analog is asymmetric dimethylarginine (ADMA), a compound that has been found in human plasma and urine and exerts the activity of an endogenous inhibitor of NO synthase. In contrast to ADMA, its regioisomer symmetric dimethylarginine (SDMA) does not inhibit NO synthase. The methyl groups contained within the dimethylarginine molecules are derived from S-adenosylmethionine, an intermediate in the homocysteine/methionine pathway. There is experimental evidence that homocysteine may affect endothelium-dependent vascular function by increasing the formation of ADMA. Both ADMA and SDMA are eliminated from the body by renal excretion. In addition, the metabolism of ADMA, but not SDMA, occurs via hydrolytic degradation to citrulline and dimethylamine by the enzyme dimethylarginine dimethylaminohydrolase (DDAH). Data from experimental studies suggest that ADMA inhibits vascular NO elaboration at concentrations found in pathophysiological conditions (i.e., 3-15 microM). ADMA likely acts as an autocrine regulator of endothelial NO synthase activity. When rabbits are placed on a diet enriched with 1% cholesterol, ADMA levels are increased within 4 weeks of dietary intervention as compared to control animals. Elevated plasma concentrations of ADMA are also present in hypercholesterolemic and hypertensive patients, in patients with chronic heart failure, and in other patient groups at high risk of developing cardiovascular disease. Elevation of ADMA induces dysfunction of the endothelium, which becomes clinically evident by impaired endothelium-dependent vasodilation, hyperaggregability of platelets, and enhanced monocyte adhesion. Recent prospective studies suggest that endothelial dysfunction indicates an increased risk of future cardiovascular events. In line with these observations, we and others found evidence that ADMA is a novel cardiovascular risk factor.