We have examined the lungs of eleven species of newborn mammals by quantitative morphometric techniques and related the findings to body weight (BW) and O 2, consumption (V̇ O 2 ). V̇ O 2 is found to be proportional to BW 0.88; this exponent is significantly greater than the value of about 0.73, repeatedly found for adult mammals. Lung volume (LV) is essentially proportional to BW, and neonatal alveolar size — as indicated by mean chord length — is nearly constant among species and independent of BW. Thus the respiratory surface area (SA) is not proportional to V̇ O 2 , as in adult mammals, but to V̇ O 2 1,2.3. Neonates of small species have lower SA/V̇ O 2 ratios than adult mammals, possibly owing to constraints of surface forces on alveolar size. Newborn members of larger species have SA/V̇ O 2 values in excess of the adult range. Published data on human newborn lung dimensions suggest that the human infant may have an unexpectedly low SA/V̇ O 2 value in relation to BW. Whether this is a valid finding, or whether it derives from methodological differences between studies cannot be ascertained from the available data.