The intrarenal interstitial pressure was measured during the course of experimental renal hypertension in dogs. In perinephritic hypertension, produced by wrapping the kidney in a cellophane bag, the intrarenal pressure rose slowly from the normal value of 25 mm. Hg to a final level of about 60 mm. Hg. Strong pressure pulsations were observed in the renal parenchyma during this type of hypertension. In the hypertension following partial occlusion of the renal artery, the intrarenal pressure remained approximately normal, except in malignant hypertension when it tended to decline to about 9 mm. Hg. The hypertension of perinephritis is interpreted as a consequence of renal ischemia, the high intrarenal pressure, produced by the constricting fibrotic hull, acting to reduce the effective perfusion pressure of the kidney. The two experimental hypertensions herein examined are considered as examples of influent resistance hypertensions and effluent resistance hypertensions, the former being due to renal arterial or arteriolar resistance and the latter due to renal venous resistance, specifically at the arcuate-interlobar junction. The application of this concept to renal hypertensive disease in man is discussed.