Currently accepted theories of the urine concentrating mechanism of the mammalian kidney predict that concentrating ability should increase with increasing length of the loop of Henle. However, larger mammals have longer nephrons than do smaller ones, yet concentrating ability declines with increasing body mass (M, in kg) as M -0"°97. Greenwald & Stetson (1988, News Physiol. Sci. 3, 46-49) have suggested that the diminished concentrating ability of large mammals reflects their lower mass-specific metabolic rate. They propose that, because the urine concentrating mechanism depends upon the energy-dependent transport of sodium chloride, concentrating ability should be closely related to mass-specific metabolic rate. Examination of the allometric scalings with body mass of medullary thickness and metabolic rate indicate that the rate of increase in length of the loop of Henle with body size (M °'129) is insufficient to offset the decline in mass-specific metabolism (M-°'24). The residual product of these scalings (M -°tl) indicates that urine concentrating ability should be inversely related to body size and is similar to the observed allometry of concentrating ability (M-°'°°7). The decline in concentrating ability of the kidney with body size is probably not a result of inability of the kidney to adapt physiologically or structurally to changes in size, but rather reflects the scaling of the need to conserve water. Small mammals, because of their high rates of evaporative and respiratory water loss, have a much higher rate of water turnover than do large mammals ( Vwater " kg -~ oc M -°2°~. Because the need to concentrate the urine diminishes with increasing body size, the increase in loop length need only partially compensate for the simultaneous decline in metabolism.