Static lung volumes, pressure-volume curves, airway conductance (Gaw) and maximum expiratory flow-volume curves were obtained in a group of healthy children and adolescents (age 6 to 17 years, body height 115 to 180 cm). Relating lung recoil pressure to maximum expiratory flow at the same lung volume level allowed the calculation of the conductance of the upstream segment (Gus). Gaw increased with increased body height and lung volume, so that specific Gaw did not change during growth. Gus also increased with body height, except at lower lung volume, but when corrected for simultaneous changes in lung volume it decreased. These physiological results suggest that the central airways grow proportionately to the changes in lung volume, whereas the growth of the peripheral airways is proportionately less marked. It is however not possible to draw definite conclusions on the isotropical or anisotropical growth of different parts of the lungs from these physiological observations.