The action of serotonin (5-Hydroxytryptamine) on the excretory function of the kidney was investigated in the dog, utilizing the clearance method and the stop-flow technique. It was shown that serotonin, 10μg/kg/min, i. v., exerts a marked antidiuretic effects and elicits a marked hemodynamic changes in the kidney: a highly significant decrease of the glomerular filtration rate and a tendency of decrement in the renal plasma flow. Little change in the systemic blood pressure was noted, and the participation of the antidiuretic hormone in the antidiuretic action was ruled out by adding vasopressin to the infusion fluid. The stop-flow analysis showed that there is no evidence of altered activity in the tubules by serotonin. It was thus concluded that serotonin elicits anti diuresis in the dog by decreasing glomerular filtration rate, which results from the constriction of Vas afferens in the glomeruli.