Endothelin is a powerful vasoconstrictor that may be partly responsible for the increases in venous and arterial tone characteristic of heart failure. The release of endothelin from endothelial cells in culture is stimulated by angiotensin II. We investigated the relationship between plasma concentrations of immuno reactive endothelin-1 and angiotensin II in 25 patients with heart failure and eight with ischaemic heart disease but normal left ventricular function. Plasma concentrations of endothelin and angiotensin II were correlated (Spearman rank correlation coefficient of 0.72; P < 0.0001) in patients with heart disease. Plasma concentrations of angiotensin II and endothelin were higher in those patients with heart failure. Angiotensin II was infused over a 3 h period in eight healthy volunteers. Infusion of angiotensin II increased plasma concentrations of angiotensin II to levels greater than those usually found in patients with severe heart failure but induced only a modest rise in plasma concentrations of immunoreactive endothelin-1 (0.77 +/- 0.16 to 1.03 +/- 0.03 pmol.l-1, P < 0.02). Increased plasma concentrations of angiotensin II and endothelin-1 both appear to reflect the presence and severity of heart failure. Although a significant correlation exists between plasma concentrations of angiotensin II and endothelin in patients with heart failure, the relationship may not be causal.