1. The ouabain-sensitive 42 K + flux from an artificial medium into erythrocytes was measured in 29 control subjects, 66 patients with chronic parenchymatous renal disease and in 32 subjects with primary hypertension. 2. The ouabain-sensitive 42 K + influx was reduced in subjects with chronic renal disease by about 20%, even when they were normotensive. The reduction in these patients was greater than that in patients with essential hypertension. 3. The changes in 42 K + influx and Na + content with a decrease in the 42 K + influx/Na + content ratio suggest an inhibition of the Na + pump in the patients with chronic renal disease. 4. The inhibition of the Na + pump may be secondary to the hypervolaemia which we suggest is the initial event leading to renal hypertension.