The scaling of the basal rate of metabolism in mammals is reexamined. Both the power and level of the scaling function are sensitive to various factors that interact with body mass and rate of metabolism, including the precision of temperature regulation, food habits, and activity level. This sensitivity implies that the rate of metabolism is a highly plastic character in the course of evolution. Consequently, the singular effect of mass on the rate of metabolism is most effectively analyzed in ecologically and physiologically uniform sets of species, rather than in taxonomically defined groups, which often are ecologically and physiologically diverse. Otherwise, all fitted curves for mammals integrate a variety of competing factors, thereby reflecting the species used and denying unique analytic significance to the power in scaling relations. Kleiber's eutherian curve may represent a relatively uniform set of data because all the species included were domesticated and because selection for high rates of production (and high rates of metabolism) occurred in the process of domestication. In the analysis of scaling relationships, the standard error of estimate (Sy.x) is a more valuable measure of the residual variation than is (1.0-r2) because r2 is a non-linear measure of the conformation of data to the relation and because Sy.x, unlike r2, is independent of the units used in the scaling relationship. At present the best estimate indicates that total rate of metabolism scales proportionally to approximately m0.60 at small masses (less than 300 g), as long as small species do not enter torpor, and scales proportionally to approximately m0.75 at large masses (greater than or equal to 300 g). Physiological properties other than metabolism are potentially sensitive to secondary factors, so their scaling functions also would be most clearly defined for physiologically uniform groups of species. This view suggests that insight into the significance of scaling relations can be obtained by examining the residual variation around a scaling function as well as by examining conformation to the function.