1. Twenty-three peritoneal dialyses with fluid containing 45 mEq lactate per litre were carried out on six patients with acute or chronic renal failure. During dialysis arterial blood pH and base excess rose. 2. The lactate ions were rapidly and almost completely absorbed from the fluid in the peritoneal cavity. Blood lactate concentration rose, but in patients with adequate liver function it did not exceed the normal range. One patient with poor hepatic function and renal failure showed abnormally high blood lactate levels after peritoneal dialysis, but metabolic acidosis was still corrected. 3. The concentration of bicarbonate ions in the fluid drained from the peritoneal cavity rose as the dialysis progressed. A significant positive correlation was found between the arterial blood bicarbonate concentration and the bicarbonate concentration in the fluid drained from the peritoneal cavity. 4. If the lactate ions absorbed from the peritoneal cavity had not been metabolized the loss of bicarbonate ions in the fluid drained from the peritoneal cavity would have increased the metabolic acidosis.