In a patient with typical features of Bartter's syndrome, intrarenal reabsorption of sodium and water at different sites of the nephron, during maximal water diuresis, was studied twice in the course of the disease. During the first study, chloride reabsorption at the loop of Henle and renal diluting ability were not impaired. Chloride reabsorption at the loop of Henle was 0.72 (normal values 0.66-0.90), minimal urine osmolality 71 mosm/kg (normal values less than 75 mosm/kg) and chloride fractional clearance 0.44 (normal values 0.3-6.8). An impaired chloride reabsorption at the loop of Henle was found 6 months later. Chloride reabsorption at the loop of Henle was 0.28, minimal urine osmolality 197 mosm/kg and chloride fractional clearance 8.23. These data suggest that the defect in chloride reabsorption at the loop of Henle is not a pathophysiological abnormality of the syndrome because it appeared at a later time than the other symptoms.