The postnatal development of the mammalian lung, which at birth has only a fraction of the adult number of alveoli, would suggest a mismatch (dysanapsis) of the changes in airway size and lung size with growth. This may have implications on the efficiency of breathing because for any given lung size the dimensions of the airways are the determinant of the amount of dead space (VD) ventilation and of airflow resistance. A comparison of the allometric functions of tracheal dimensions previously published suggests that the tracheal volume, taken as representative of VD, is smaller in newborns than in adults. This difference becomes more apparent when examined per unit of functional residual capacity (VD/FRC). The relatively smaller tracheal volume is the result of both a shorter and narrower trachea in the newborn. This latter difference implies a slightly but significantly higher resistance of the lower airways, as also demonstrated by experimental measurements of peak expiratory flows at a constant driving pressure in rats of different ages. Because the slightly higher resistance of the lower airways is probably compensated by the smaller resistance of the upper airways (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 51: 641-645, 1981), it would seem that the structure of the newborn mammalian lung favors the alveolar ventilatory function without a substantial increase of the energetic losses.