Chronic heart failure (CHF) is associated with neurohumoral activation and alterations of the peripheral circulation. Several mechanisms are involved in the impaired peripheral perfusion, including increased sympathetic tone. Recent data suggest an important role of the endothelium for peripheral perfusion in CHF. Endothelium-dependent dilation of resistance vessels is blunted in patients with severe CHF and may be involved in the impaired reactive hyperemia in these patients. In conductance vessels, flow-dependent dilation is reduced in CHF, reflecting endothelial dysfunction of large conduit vessels. To investigate endothelial function in humans in vivo, agents such as acetylcholine are used to stimulate the release of endothelium-derived nitric oxide (NO). Conversely, N-mono-methyl-L-arginine (L-NMMA), a specific inhibitor of NO synthesis from L-arginine, decreases forearm blood flow by inhibiting the basal release of NO. Conversely to the impaired stimulated release of NO by acetylcholine, the decrease in blood flow induced by L-NMMA appears to be exaggerated in CHF. The blood flow response to nitroglycerin or sodium nitroprusside, both endothelium-independent vasodilators, is usually preserved in patients with nonedematous CHF, indicating a normal response of the vascular smooth muscle of resistance vessels to exogenous NO. Therefore, impaired endothelium-dependent dilation of peripheral resistance vessels emerges in CHF, suggesting a reduced release of NO on stimulation. Endothelial dysfunction may therefore be involved in the impaired vasodilator capacity in the peripheral circulation, e.g., during exercise. In contrast, the basal release of NO from endothelium of resistance vessels appears to be enhanced and may play an important compensatory role in CHF.