Beta-adrenergic blockade may impair the normal cardiovascular response to hypoxia occurring during general anaesthesia. The haemodynamic effects of acute hypoxia, induced by a 90-s period of ventilation with nitrogen, were studied during increasing depths of halothane anaesthesia up to a maximum of 2.5% inspired halothane in dogs chronically implanted with intracardiac catheters, a left-ventricular pressure transducer and an aortic blood flow transducer. An untreated group of dogs and a group which had been treated for 3 weeks with propranolol 20 mg/kg/day were compared. The beta-blocked group had lesser cardiac output values, left-ventricular contractility indices, external left-ventricular work and peak left-ventricular power at all depths of anaesthesia except 2.5% halothane, but both groups responded to hypoxia similarly at each depth of anaesthesia. Cardiac performance was enhanced in both groups during acute hypoxia. No adverse haemodynamic effect of the combination of propranolol, halothane and hypoxia was demonstrated.