The effects of isoproterenol and dopamine on regional myocardial blood flow was studied in ten open-chest dogs after proximal circumflex coronary artery stenosis. Regional myocardial flows were determined with radioactive microspheres. Both drugs increased mean aortic flow and myocardial contractility. Isoproterenol caused relative subendocardial ischemia in the myocardium with compromise coronary flow. Dopamine did not produce regional myocardial ischemia. It is concluded that dopamine is the preferable inotropic drug in treating low cardiac output in patients with coronary artery disease.