Urotensin II has vasoconstrictive and negative inotropic effects, suggesting a possible role in circulatory regulation and pathophysiology of heart failure. We developed a sensitive specific RIA and measured plasma urotensin II in patients with heart failure and in controls. Plasma urotensin II was higher in heart failure patients (mean 3.9 pmol/L [SD 1.4]; than in controls (1.9 pmol/L [0.9]; p<0.0001). The role of urotensin II in heart failure, however, has yet to be defined.