B. H. Blake (Comp. Biochem. Physiol. A Comp. Physiol. 58: 413-419, 1977.) and W. A. Calder and E. J. Braun (Am. J. Physiol. 244 (Regulatory Integrative Comp. Physiol. 13): R601-R606, 1983.) have predicted that urine concentrating ability of mammals should decline with increasing body mass (M, in kg) as M-0.08. Edwards (29), on the other hand, speculated that concentrating ability should be independent of body mass. Using information compiled from the literature for 245 species of mammals, I examined the scaling of urine concentrating ability with body mass. Maximum urine concentration (Uosm, in mosmol/kgH2O) declined exponentially with body mass as Uosm = 2,564 M-0.097, and generally only the smallest species (less than 400 g) could produce urine with an osmolality greater than 4,000 mosmol/kgH2O. Medullary thickness (MT, in mm) and, therefore, the length of the loop of Henle, increased with body mass as MT = 8.147 M0.129. The thickness of the medulla relative to the size of the kidney (RMT), however, declined with body size (RMT = 5.408 M-0.108). The relative thickness of the medulla accounted for only 59% of the variability among species in concentrating ability, indicating that there are other morphological or physiological factors that significantly influence urine concentrating ability.