This report has reexamined the relationship between standard and maximal rates of oxygen consumption (VO2std and VO2max) and pulmonary surface area in mammals whose weights extend over the lower half of the total log weight range in mammals. For combined groups of wild and laboratory animals with body weights of 2--3700 g, the following equations pertain: VO2std (ml . min-1) = 0.0602 . W0.727; VO2std, or the factorial aerobic scope, is nearly constant over this weight range at approximately 6.6 (range 5.8--7.1). In view of this finding, earlier studies relating SA or pulmonary diffusion capacity to VO2std are still appropriate models for pulmonary constraints on metabolic rate. The data summarized here suggest that, at least for wild species of mammals, pulmonary diffusion capacity may limit VO2max.