1. Intravenous infusion of isoproterenol (0.1 to 0.5μg/kg/min) in the dog produced a de-crease in GFR, RPF and urine volume, accompanied by a fall in systemic blood pressure. Urinary excretion of sodium, potassium and chloride significantly decreased. A dose of 0.5μg/kg/min resulted in a significant decrease in the urinary concentrations of these electrolytes, but a dose of 0.1μg/kg/min resulted in no change. Osmolal clearance decreased but free water clearance increased during the isoprotenol infusion. 2. When 0.01 to 0.05μg/kg/min of isoproterenol was infused into the left renal artery, GFR and RPF of the infused kidney increased with no changes in systemic blood pressure, while those of the opposite kidney remained unchanged. Being accompanied with diuresis, there developed an increased excretion of sodium, potassium and chloride in the in-fused kidney. Osmolal clearance increased, free water clearance decreased. These changes, but however, were not observed in the opposite kidney. 3. It is concluded that isoproterenol has a diuretic effect through its direct action on renal function and that the antidiuresis induced by this drug is mainly due to its systemic hemodynamic effect.