1. Intravenous infusion of l -glutamic acid results in the augmentation of the cardiac output and an improvement of the circulation in patients with postoperative cardiac failure. This effect is not accompanied by increased myocardial oxygen demand. 2. Arterial plasma glutamate level rises 10-fold and arterial-coronary sinus plasma glutamate difference increases fivefold during intravenous l -glutamic acid infusion. This leads to cessation of ammonia release from the myocardium, probably due to augmentation of glutamine synthesis and to an increase in alanine formation coupled with a change from lactate release to lactate uptake by the myocardium. 3. The data obtained suggest that the beneficial effect of l -glutamic acid on depressed cardiac function in postoperative patients is related to changes in myocardial metabolism. Glutamic acid may be useful in treatment of circulatory and metabolic disturbances in cardiac failure.