Transpulmonary pressure, air flow, and end-tidal carbon dioxide levels were measured in normal human volunteers during hypocapnic, eucapnic, and hypercapnic hyperventilation. Respiratory rate and tidal volumes were well matched at a minute ventilation of 52 L. on three inspired gas mixtures: 21 per cent oxygen and 79 per cent nitrogen; 5 per cent carbon dioxide, 21 per cent oxygen and 74 per cent nitrogen; and 12 per cent carbon dioxide, 21 per cent oxygen and 67 per cent nitrogen. Respiratory rate, tidal volume, lung compliance, resistance, and resistive work per liter were calculated with a digital computer. In 13 experiments in 7 normal volunteers, no net bronchoconstriction or bronchodilatation was observed when eucapnic hyperventilation was compared to hypocapnic or hypercapnic hyperventilation. During hyperventilation of this degree, a change in bronchomotor tone owing to alteration in arterial or alveolar PCO2 either does not occur or else is masked by other reflexes or mechanical factors acting on the bronchi.