An impairment in renal concentrating ability can be easily recognized by the maximum concentrating ability (Umax) as determined by the urine osmolality reached after a fixed period of dehydration. Inability to maximally concentrate the urine is related to one or both of two basic tubular defects: 1) a failure of maximal free-water generation by the diluting segment in the ascending limb of Henle's loop; and 2) a failure of the distal tubular epithelium to achieve maximum permeability to water during water deprivation.