1. Ouabain-sensitive ATPase (adenosine triphosphatase) activity was measured in the erythrocyte membranes of twenty patients with chronic renal disease. Decreased activity was found in nineteen of the twenty patients. The average decrease was 38%.
2. In three patients erythrocyte sodium concentration exceeded 9·5 mmol/l of RBC and these patients had the most marked decreases in ouabain-sensitive ATPase activity.
3. By contrast, in only two of 100 patients admitted to a general medical ward was erythrocyte sodium concentration greater than 9.5 mmol/l of RBC.
4. Ouabain-insensitive and calcium-activated components of erythrocyte membrane ATPase were unaffected by chronic renal disease.