The intravenous administration of 5.0 mg of glucagon to 16 digitalized patients on the first postoperative day after prosthetic valve replacement resulted in significant cardiac inotropic effects. Since glucagon is effective in the presence of catecholamine depletion or beta-adrenergic blockade and appears to have little arrhythmogenic potential, it would appear to be a useful agent for the treatment of myocardial depression occurring in association with open cardiac surgery.