SUMMARY The recoveries from portal blood of Pitressin and Azovan blue injected into the superior mesenteric artery in neurohypophysectomized rats were determined. The ratio of the recoveries of Pitressin and Azovan blue was 1·00±0·15. In rats with intact neurohypophyses and anaesthetized with urethane, the concentration of antidiuretic hormone in arterial blood was not different from that in portal blood. It is therefore concluded that vasopressin and endogenous antidiuretic hormone are not cleared rapidly from the circulation in the intestines. The pressor responses to 20 mu. Pitressin given intraportally were smaller than those obtained with 10 mu. given intravenously, suggesting rapid clearance of Pitressin from the circulation in the liver. A technique is described for determining the renal clearance of test substances infused intravenously into conscious rats followed by sampling of the arterial blood. In six rats the urinary clearance of Pitressin was 1·24±0·06 times the clearance of inulin. In two rats in which urine flow was unusually low, clearance of Pitressin was respectively 0·161 and 0·325 times the inulin clearance. It is calculated that the rate of clearance of Pitressin from the circulation in conscious rats corresponds to a half-life of 42 sec.