The influence of guanethidine on the renal function was investigated in the rabbit. Guanethidine, 1-10mg/kg, i.v., produced no marked change in the renal function, while second and successive doses of guanethidine elicited a significant increase in urine flow and electrolyte excretion as well as renal plasma flow and glomerular filtration rate. It was suggested that the diuretic action was brought about by improvement of hemodynamic state in the kidney ; increased filtration as a result of increased renal perfusion. Atropine alone did not significantly influence the renal function but pretreatment of animals with atropine, 4 mg/kg i.v., completely abolished the diuretic action of guanethidine. It is suggested that guanethidine influences the renal function by activating parasympathetic nervous system or some cholinergic mechanism in the kidney.