Isoproterenol was effective in increasing cardiac output and in reducing left-ventricular end-diastolic pressure in virtually all forms of heart disease, with the exception of some cases of severe coronary-artery disease or aortic stenosis. Because of these physiological observations, isoproterenol was used in the acute treatment of certain expressions of circulatory failure. Isoproterenol was useful in potentiating mercurial diuresis, and occasionally was an effective potentiator when aminophylline was not. Isoproterenol was particularly helpful in treating either bradycardia or hypotension when these complicated congestive failure or the postoperative low output state.