ney or the renomedullary transplants was followed by a prompt rise in arterial pressure and death of the animal. Thus, the antihypertensive action of renomedullary tissue was similar to that of the whole kidney. The main cell type noted in the protective renomedullary transplants had the microscopic characteristics of the lipid-containing interstitial cells. These cells occurred This work was presented in part at the Meeting of the American Physiological Society, Atlantic City, N. J., 14 April 1970, and the Southern Society of Clinical Investigation, New Orleans, La., 30 January 1971. This study appeared in abstract form, Fed. Proc. 1970. 29: 447, and Clin. Res. 1971. 19: 65. Received for publication 26 April 1971 and in revised form 4 August 1971. in clusters, often were near capillaries, and appeared hyperplastic. It is suggested that the renomedullary interstitial cell is the most eligible cell for exertion of the renomedullary antihypertensive action. Since vasoactive lipids are extractable from the renal medulla and its interstitial cells, the hypothesis that interstitial cells secrete antihypertensive substance(s) is attractive.