The effects of infusing human alpha-calcitonin gene-related peptide were studied in eight patients with congestive heart failure, five normal rabbits and five rabbits with adriamycin-induced cardiomyopathy. In patients with heart failure, calcitonin gene-related peptide caused a dose-dependent increase in cardiac output and decrease in pulmonary and systemic vascular resistance and pulmonary artery pressure. The systemic blood pressure and right atrial and pulmonary wedge pressures decreased only at the highest infusion rate (16 ng/kg per min). Heart rate remained unchanged. Plasma epinephrine increased (p less than 0.05), whereas aldosterone, atrial natriuretic peptide and prolactin concentrations decreased (p less than 0.05). Plasma norepinephrine, renin activity, cortisol and growth hormone concentrations remained unchanged. In both groups of rabbits, the drug decreased blood pressure and increased cardiac output and heart rate. There was a significant increase in renal blood flow (p less than 0.05). The peptide did not affect the contraction amplitude of human and rabbit ventricular myocytes. These findings suggest that calcitonin gene-related peptide is a vasodilator in the rabbit and humans with little direct effect on ventricular myocardium. This peptide may be useful in some forms of heart failure.