1. Endothelium-dependent vasodilatation via nitric oxide in response to muscarinic stimulation is decreased in chronic heart failure while basal release of nitric oxide may be increased. As production of the endothelium-derived vasoconstrictor endothelin-1 is increased in chronic heart failure, endothelin-1 may act in an autocrine manner to modulate these effects. 2. To test this, we determined whether prolonged endothelin infusion in normal subjects would reproduce the alterations in basal and stimulated nitric oxide release observed in patients with chronic heart failure. Basal nitric oxide production was determined by measurement of forearm blood flow using strain gauge venous occlusion plethysmography before and after brachial artery infusion of a nitric oxide synthase inhibitor (NG-monomethyl-L-arginine). Stimulated nitric oxide production was determined by brachial artery infusion of acetylcholine. As metabolic vasodilatation is thought to be mediated in part via nitric oxide and is decreased in chronic heart failure, forearm blood flow during peak reactive hyperaemia was also measured. Studies were then repeated during brachial artery infusion of endothelin-1 and a non-specific vasoconstrictor, noradrenaline. 3. Neither basal nor stimulated nitric oxide production was altered by endothelin-1 and noradrenaline infusion. However, absolute forearm blood flow responses to peak reactive hyperaemia were decreased during infusion of endothelin-1 in comparison to noradrenaline. These data suggest that increased endothelin-1 may not contribute greatly to altered basal and stimulated nitric oxide production in patients with chronic heart failure but may contribute to impaired metabolic vasodilatation, by mechanisms presumably unrelated to altered nitric oxide production.