1. Erythrocyte metabolism was investigated and glycolytic intermediates were measured in nine patients with chronic renal failure who were subsequently treated with haemodialysis. The same' investigations were performed in nine patients on continuous ambulatory peritoneal dialysis (CAPD) who had previously been treated with intermittent peritoneal dialysis (IPD) (eight patients) or dietary restriction (one patient). 2. The patients who received haemodialysis had a partially compensated metabolic acidosis before treatment. With haemodialysis, plasma phosphate (P i ) fell and base excess, erythrocyte 2,3–diphosphoglycerate (2,3–DPG), glucose consumption and lactate production rose significantly. In this group the most important influence on erythrocyte metabolism was base excess. The pattern of erythrocyte glycolytic intermediates showed that the rise in 2,3–DPG with haemodialysis was brought about within the Rapoport—Luebering shunt; there was no statistically significant decrease in haemoglobin—oxygen affinity. 3. The patients who received CAPD were not acidotic before starting this form of treatment. With CAPD, there was a significant increase in haemoglobin and fall in plasma phosphate, erythrocyte 2,3–DPG and glucose consumption. The major factors influencing erythrocyte metabolism in this group were plasma phosphate and haemoglobin concentration. The fall in 2,3–DPG was produced by inhibition of 6-phosphofructokinase (EC 2.7.1.11); despite this fall, haemoglobin-oxygen affinity was not affected.