This study was attempted to investigate the action of debrisoquine, a sympathetic blocking agent presently employed in treating hypertension, on renal function and to elucidate the mechanism of its action. Debrisoquine, given intravenously, elicited increased urine flow, osmolar and free water clearances, along with marked increases in excretion of both sodium and potassium. Glomerular filtration rate also increased, but renal plasma flow tended to decrease, so that the filtration fraction tended to increase. Rates of reabsorption of sodium and potassium in renal tubules were also significantly diminished. The diuresis induced by debrisoquine was completely blocked by treatment with phentolamine and reserpine, and also markedly inhibited by acute renal denervation. Debrisoquine, when injected directly into a renal artery, produced antidiuretic effect and a reduction in urinary excretion of sodium and potassium, along with diminished renal plasma flow and increased filtration fraction. The above observations indicate that debrisoquine, when given intravenously, induces diuresis in the dog as a result of both diminished tubular reabsorption of electrolytes and of renal hemodynamic changes, which seem to be related to its inhibitory action of catecholamine-release from the sympathetic nerve endings.