1. The pattern of urinary acid excretion during mannitol diuresis was studied in five normal adult subjects.
2. At peak diuresis (osmolal clearance 19·3–27·9 ml/min), the urinary pH approached 6·9 irrespective of the initial pH.
3. Total hydrogen ion excretion fell sharply during mannitol diuresis. Changes in the excretion of ammonium and titratable acid were small and variable, but in each subject bicarbonate excretion increased markedly.
4. Despite previous conflicting published results urinary acid excretion during mannitol diuresis in man resembles that in the dog and is dominated by a bicarbonate diuresis.