Abstract Anesthetized mongrel dogs were paced at constant heart rates after creation of complete A-V block by formalin injection. The circumflex artery was narrowed by 50 to 90 per cent using a variable occlusion clamp. Arterial and coronary sinus pH, P o 2, P co 2, lactate and pyruvate levels, aortic and right atrial pressures, aortic and circumflex artery flows, aortic dP dt , and electrocardiogram Lead II were monitored. Recordings were made before and after circumflex narrowing, during and after isoproterenol infusion, 3 mcg. per minute (7 animals); or during and after glucagon infusion, 35 mcg. per minute (7 animals, pre-blocked with propranolol). Isoproterenol caused positive inotropic changes associated with a marked decrease in myocardial lactate extraction (−37.4 per cent), and a decrease in mean aortic pressure. Glucagon infusion caused similar changes of increased inotropy and a decrease in lactate extraction; however, there was no significant change in aortic pressure. It is concluded that in the presence of restricted coronary arterial inflow use of the potent inotropic agents isoproterenol and glucagon may result in progressive deterioration due to excessive energy utilization induced by these agents.