1. The ouabain-sensitive 42K+ 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 42K+ 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 42K+ influx and Na+ content with a decrease in the 42K+ 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.