Hypertonic sodium chloride and tris(hydroxymethyl)amino-methane (Tris) hydrochloride (0.3 mmol/kg.min) were infused intravenously into chloralose-anesthetized dogs over a 20-min period. Cardiac output, maximum left ventricular dP/dt, and (dP/dt)/P increased by 55, 33, and 23%, respectively, during NaCl infusion, but Tris HCl infusion had no effects. NaCl infusion did not change heart rate or left ventricular end-diastolic pressure. Mean systemic and pulmonry arterial blood pressures increased, whereas total peripheral and pulmonary vascular resistances decreased. Responsiveness of cardiac beta-adrenergic receptors, as determined by serial intravenous injections of epinephrine, was not affected by NaCl infusion. Plasma catecholamine concentration, however, increased during NaCl infusion. In addition, the increases in cardiac output, maximum left ventricular dP/dt and (dP/dt)/P were abolished by prior treatment with practolol, a cardioselective beta-adrenergic receptor blocking agent. These results suggest that the hemodynamic effects of NaCl infusion were caused, at least in part, by the inotropic action of catecholamines.