Studies of clinical and hemodynamic changes in 13 patients with shock due to acute myocardial infarction were conducted. Both levarterenol (norepinephrine) bitartrate and isoproterenol hydrochloride were infused, and the clinical and hemodynamic effects were compared. Arterial pressure could be increased to nearly normal levels in all patients when levarterenol was infused, and this drug appeared to contribute to recovery in four patients. Adequate arterial pressure was obtained in only one patient during isoproterenol infusion, and clinical deterioration was coincident with the change from levarterenol to isoproterenol infusion in four patients. Increases in cardiac output occurred with both drugs, but was related to clinical improvement only when this increase was associated with an elevation in mean arterial pressure. Maintenance of an adequate arterial perfusion pressure is essential in treating shock due to acute myocardial infarction. In most patients, levarterenol is superior to isoproterenol in achieving this goal.