Two syndromes are called syndromes X: cardiac (effort anginal pain, positive exercise tolerance test and absence of angiographically documented critical stenosis in coronary arteries) and metabolic (according to WHO definition: impaired glucose tolerance and insulin resistance and > or = 2 risk factors from the following list: hypertension, dyslipidaemia, visceral obesity and microalbuminuria). Hyperinsulinaemia and endothelial dysfunction are present in both syndromes. The contribution of endothelial nitric oxide synthase gene mutations to the etiology of these syndromes is also studied. Several mechanisms may be involved in the development of endothelial dysfunction, such as reduced synthesis and release of nitric oxide (NO), enhanced inactivation of NO after its release from endothelial cells or enhanced synthesis of vasoconstricting agents. It has been demonstrated that insulin exerts a direct hypertrophic effect on the vascular endothelium and the smooth muscle cells. The hemodynamic properties of insulin have also been discussed. Some findings suggest that in the skeletal muscle circulation, insulin stimulates both endothelin-1 (ET-1) and nitric oxide activity and an imbalance between the release of these two substances may be involved in the pathophysiology of endothelial dysfunction.