Alveolar gas is compressed and expanded during every breathing cycle. The volume displacement measured at the mouth Vt (tidal volume) is therefore smaller than the volume displacement of the lung V't that may be measured with a body plethysmograph. Experimental data found in normal subjects and in patients with obstructive emphysema confirm the theoretical prediction that the ratio Vt/V't decreases with increasing airway resistance, breathing frequency, and lung volume. The effect was found to be very small in normal subjects breathing at different breathing rates; however, in patients with obstructive emphysema it may be appreciable, presumably because of the high airway resistance and the elevated functional residual capacity. With increasing altitude the effect is expected to be more pronounced. The mechanical work performed in compressing and expanding alveolar gas is not included in the conventional pressure-volume diagram, when intraesophageal pressure is plotted against the volume displacement measured at the mouth. This work may be determined, however, by recording the volume displacement of the lung simultaneously with tidal volume and intraesophageal pressure. Work related to compressibility is insignificant in most circumstances. In patients with obstructive emphysema, however, it becomes appreciable during hyperpnea. The compressibility of alveolar gas was also found to increase the negative work performed by respiratory muscles. volume displacement of the lung (V't); Vt/V't in obstructive emphysema; dynamic pressure-volume diagram; mechanical model - respiratory dynamics; ventilation; negative work of inspiratory muscles; ventilation and breathing work at high altitudes; mechanics of breathing Submitted on June 21, 1963