Inotropic responses to digoxin (0.08 mg/kg) were studied in dogs and compared with responses during hypoxemia and autonomic blockade. Changes in left ventricular contractility (VC) were assessed by constructing function curves relating left ventricular (dP/dt)max and stroke volume to end-diastolic pressure. Augmentation of VC was observed 20 min after digoxin infusion and continued to increase until termination of the experiment after 60 min. In animals subjected to autonomic blockade with practolol, TEAC, and atropine, the increases in VC after digoxin were substantially greater. Equally large increases occurred in blocked dogs during sustained hypoxia (Pao2 = 28 mmHg). However, in animals without blockade there was a progressive fall in VC during hypoxia despite digoxin infusion, although less than in those not given digoxin. Serum digoxin levels were measured by radioimmunoassay and did not differ significantly in blocked compared to unblocked dogs or in hypoxic compared to nonhypoxic animals. These findings indicate that digoxin protects the heart from the decrease in myocardial contractility which occurs during extended hypoxia. This protective effect is more pronounced in animals deprived of autonomic function, possibly reflecting the elimination of reflex sympathetic withdrawal ordinarily induced by digitalis.