A 200 Hz tone from a sinewave generator was utilised to monitor respiration. The sound was led into the lung from the mouth and the transmitted sound was detected through the thoracic cage by a phonocardiogram microphone fixed on the chest wall. The microphone output voltage, represented by its absolute mean value, varied with the respiration of the subject. Compared with the inspiratory or expiratory gas volume obtained by a conventional method, it was shown that the variation in the output was proportional to the change in pulmonary gas volume. With our acoustic pneumograph, it was possible to measure not only the change in pulmonary gas volume but also the variation in the end-expiratory level. The mechanisms of the changes in the output and the linearity of this device were also examined.