Lactic acidosis accompanied by acute renal failure in the newborn period was studied in two infants with circulatory insufficiency and hypoxia. Peritoneal dialysis was necessitated by anuria and serum potassium concentrations of 12.0 and 8.9 mEq/l. Plasma lactate concentration was 35 and 50 mM/l and blood pH 7.23 and 7.18, respectively, at the time dialysis was instituted. Because of the uncontrollable anaerobic metabolism in these two patients, and the attendant inability to metabolize lactate, the use of commercial lactate-containing dialysates as a source of base was shown to be ineffective in correcting the acidosis and hypothesized to cause a worsening of metabolic acidosis due to a loss of bicarbonate from extracellular fluid into dialysate. Stabilization or improvement in the metabolic acidosis occurred with the utilization of a dialysate containing bicarbonate with a gradient favoring movement of bicarbonate into, and lactate out of, extracellular fluid.