A method is described for preparing and studying the acute effect of renal transplants on the blood pressure of the rat. In such a preparation, renal blood flow, urine flow, and blood pressure are well maintained over a period of several hours. In hypertension of less than 6 months' duration following renal ligation or desoxycorticosterone administration, the transplant consistently reduced the blood pressure and in 36 out of 44 experiments normotensive levels were achieved in 30 to 40 minutes. Exclusion of the kidney permitted a gradual restoration of the hypertension. When the blood pressure was elevated by infusions of renin or angiotensin, the blood pressure did not fall after the transplantation procedure. Introduction of kidney tissue after the hypertension had lasted for 6 months or more reduced the blood pressure in only 10 of 23 experiments, suggesting that after the passage of time irreversible vascular changes had occurred.The mechanism by which the transplant lowers the blood pressure is unknown, but such factors as the creation of an area of low vascular resistance, the release of non-specific depressor substances into the circulation, and the external loss of a pressor substance into the urine seem to be excluded. Reasons are given for suspecting that the antihypertensive action of normal kidney tissue is a specific metabolic or incretory effect, rather than an excretory action related to urine formation.