1. Simultaneous measurements of the tidal volume and the intra-oesophageal pressure were made in fifteen hypoxaemic patients with chronic bronchitis during periods of air breathing and 30% oxygen breathing; values of dynamic lung compliance were obtained.
2. During the period of oxygen breathing, the dynamic lung compliance increased in fourteen of the patients (the mean increase was 28%).
3. The increase in dynamic lung compliance could not be accounted for by a change in the frequency of breathing, tidal volume or minute volume.
4. It is concluded that the increase in dynamic lung compliance reflected a reduction in airways resistance during oxygen breathing. It is suggested that this could lead to a change in the physiological dead space in the absence of any change in the overall ventilation of the lungs.