(1) Concentrating ability was studied in rats with experimental pyelonephritis. Maximal urinary concentration was shown to be reduced as early as one week after introduction of infection and persisted unchanged up to 21 weeks with repetitive injections of bacteria. (2) Glomerular filtration rate was unaltered throughout the study period. PAH clearance fell in the repetitively injected animals. (3) Sodium content of the renal papilla remained normal but urea content fell in the one-week singly injected and 21-week repetitively injected animals. (4) Histological examination showed progressive dilatation of residual collecting ducts. (5) It is suggested that the concentrating defect in early experimental pyelonephritis is due to impaired permeability of the collecting ducts to urea and water.