Nine patients with alcoholic cardiomyopathy and 8 patients with mitral stenosis were studied by right and left heart catheterization. Hemodynamic observations were made during control period and during the infusion of phentolamine at a rate of 0.3 mg per minute. The patients with the cardiomyopathy responded to phentolamine with a significant decline in the left ventricular filling pressure, pulmonary artery mean pressure, peripheral resistance, and arteriovenous oxygen difference. There was a significant increase in the cardiac index. The patients with mitral stenosis did not show any hemodynamic improvement after phentolamine infusion.