In 11 patients on continuous ambulatory peritoneal dialysis 2,3-diphosphoglycerate, oxygen (O<sub>2</sub>) affinity of red cells (P<sub>50</sub>), blood gases, ventilation, and O<sub>2</sub> consumption were studied. Contrary to patients on maintenance hemodialysis, 2,3-diphosphoglycerate and P<sub>50</sub> were normal in patients on continuous ambulatory peritoneal dialysis; they were correlated with each other. Arterial O<sub>2</sub> pressure was normal; under the conditions of dialysis with 35 mmol lactate per liter dialysis fluid a slight metabolic acidosis persisted; it was combined with a moderate respiratory alkalosis. Position had no influence upon pulmonary gas exchange with the exception of the alveolo-arterial gradient which was elevated when the abdominal cavity was filled with dialysis fluid and the patients assumed the supine position. Ventilation was in the normal range, whereas O<sub>2</sub> consumption was low, possibly due to a reduction in muscle mass in some of the patients. It is concluded that pulmonary gas exchange and erythrocyte O<sub>2</sub> transport were normal and that O<sub>2</sub> requirements tended to be low in this group of patients on continuous ambulatory peritoneal dialysis.