A high chest wall/lung compliance ratio is an obligatory structural requirement in the newborn. A flexible chest wall is an essential prerequisite for the delivery through the birth canal, and relatively stiff lungs result from the presence of the fluid in the interstitium. A high compliance yields a long time constant, which may help in raising the end expiratory level, an important functional priority in the neonatal respiration. It is likely, however, that the goal of maintaining a high mean lung volume is mainly achieved through closure of the glottis and/or inspiratory muscle activity during expiration. A relatively long time constant during inspiration may generate some functional problems, such as volume instability to pressures applied and not ready responses to inspiratory pressure swings. Some evidence suggests that in dynamic conditions the compliance of the respiratory system is smaller than the passive value, effectively reducing the inspiratory time constant. A comparative analysis of the tracheal dimensions, matched by studies of expiratory flows at a fixed driving pressure in animals of different ages, indicate that in the newborn the trachea has a relatively smaller volume and larger air-flow resistance. This latter factor is probably offset by the smaller upper airway resistance in the newborn. If the trachea was representative of the whole dead space, it would seem, therefore, that the structure of the newborn mammalian lung favors the alveolar ventilatory function, without a substantial increase in the energetic losses.