Static mechanical properties of the respiratory system have been examined during growth. Static inflation limb pressure-volume curves were obtained in rats and rabbits of three age groups, newborn, 2 wk, and adult. Lung weight-to-body weight ratio (LW/BW) decreased with age. Functional residual capacity (FRC) decreased with age when expressed per unit BW but increased when expressed per unit LW. Elastic recoil pressure of the lung at FRC increased during growth. Respiratory system and chest wall compliance per unit body weight (Crs/BW, Cw/BW) decreased during growth while lung compliance (CL/LW) increased. Examination of the newborn and adult allometric functions with respect to each other, suggests that, in general, changes in respiratory variables during growth are age dependent rather than size dependent. The slopes of these functions during growth tend to be less than the allometric slopes found in either the newborn or adult. In the adult the respiratory system and lung-specific compliances are interspecific constants, whereas chest wall specific compliance decreases with body size. In contrast, all the specific compliances of the newborn are size independent. The allometric ratio of Cw to CL decreases with body size in the adult, whereas this ratio is larger in the newborn and is an interspecific constant.