Heart failure is commonly associated with high plasma concentrations of endothelin-1, a powerful vasoconstrictor produced by endothelium. The role of endogenously released endothelin-1 in the maintenance of vascular tone in chronic heart failure was assessed by acute administration of an endothelin receptor antagonist, bosentan. 24 patients with chronic heart failure received randomly and double blind two intravenous infusions of either placebo or bosentan (100 mg followed after 60 min by 200 mg). Systemic haemodynamics and plasma endothelin-1 and big-endothelin-1 concentrations were determined before and repeatedly during the 120 min observation period. Baseline endothelin-1 and big-endothelin-1 concentrations, which were above the normal range in all patients, correlated directly with the extent of pulmonary hypertension, with left and right heart filling pressures, and with pulmonary vascular resistance and inversely with cardiac index. Compared with placebo, bosentan reduced mean arterial pressure by 7.7% (95% CI 7.1-9.7), pulmonary artery pressure by 13.7% (10.5-16.9), right atrial pressure by 18.2% (12.0-24.4), and pulmonary artery wedged pressure by 8.6% (5.3-12.0); it increased cardiac index by 13.6% (9.1-18.2), decreased systemic vascular resistance by 16.5% (13.2-19.8), and decreased pulmonary vascular resistance by 33.2% (22.4-44.0). Heart rate did not change. Plasma endothelin-1 concentrations rose more than twofold from baseline in bosentan recipients while big-endothelin-1 concentrations were unchanged. These findings indicate that, in patients with chronic heart failure who have high circulatory endothelin-1 concentrations, this peptide contributes to maintenance of vascular tone. The acute haemodynamic effects of bosentan suggest that chronic endothelin antagonism could be beneficial in such patients.