Renal excretion of hydrogen ion has been measured in six patients recovering from acute oliguric renal failure. Despite extracellular acidosis the majority of patients were unable to lower their urine pH to the values achieved by acidotic normal subjects. This defect was not as marked as is usually seen in patients with renal tubular acidosis.Ammonium excretion was markedly reduced in all patients. Endogenous creatinine clearance was also reduced, the reduction in ammonium excretion being proportional to the reduction in creatinine clearance. As this relationship is seen in other forms of renal disease, even in the absence of signs of tubular dysfunction, it cannot be concluded that a low ammonium excretion in acute oliguric renal failure is the direct result of structural damage to the renal tubular epithelium.