In the normal state the known antihypertensive and prohypertensive functions of the kidney-excretory control of excess sodium and water on the one hand and direct pressor effects on the other-are kept in balance. But deficiency or inactivity of a normal nonexcretory antihypertensive function may also be a prohypertensive factor. Evidence is described that localizes such a function to the interstitial cells of the renal medulla.