The rats with chronic renal hypertension caused by constricting one renal artery, exhibit a decrease in the activity of Na-K-ATPase in the outer medulla of the "untouched" kidney, as compared to this activity in the kidneys of intact normotensive rats and in the "untouched" kidney of the rats where renal artery constriction did not result in hypertension. There were no differences between the control normotensive Wistar rats and the spontaneously hypertensive rats (SHR) in the prehypertensive and early hypertensive stages (at the age of 6-8 weeks) as far as the activities of Na-K-ATPase and oxidoreductases (SDH and LDH) in the renal cortex, the outer and inner medulla are concerned. The spontaneously hypertensive rats with chronic hypertension had at the age of 16-20 and 27-29 weeks lower activity of Na-K-ATPase, SDH, and LDH in the outer renal medulla than the control normotensive Wistar rats. The experimental results indicate that in chronic arterial hypertension there is a decrease in the activity of Na-K-ATPase, in the outer renal medulla, which suggests a reduction in the resorpo sodium and water.