In most circumstances in health, efficient alveolar ventilation and alveolar-to-arterial exchange of O2 and CO2 are among the strongest of links in the gas-transport chain during maximal exercise. Indeed, in most instances, the metabolic cost of ventilation represents the only significant contribution of the pulmonary system to the limitation of O2 transport of locomotor muscles and thus to the limitation of maximum performance. Of the "weaknesses" inherent in the healthy pulmonary system response to exercise, the most serious one may well be its absence of structural adaptability to physical training or to the trained state. Thus, the lung's diffusion capacity and pulmonary capillary blood volume remain unaltered in the highly trained human or horse, while maximum pulmonary blood flow rises linearly with the enhanced max VO2. Similarly, ventilatory requirement rises markedly, with no alteration in the capability of the airways to produce higher flow rates or of the lung parenchyma to stretch to higher tidal volumes, and little or no change in the pressure-generating capability of inspiratory muscles. The case of the elderly athlete who remains capable of achieving high maximum pulmonary blood flows and ventilatory requirements and whose lung undergoes a normal aging process underscores the importance of deficits (from "normal") on the capacity end of this continuum of cost versus capacity in the pulmonary system. The asthmatic athlete may represent another such example of limited flow-generating capacity; and the healthy, young, highly fit athlete who shows marked reductions in SaO2 and in max VO2 at even moderately high altitudes demonstrates that, in many situations, precious little room can be added to the demand side or removed from the capacity side before signs of failure can be seen.