A simple clinical method, allowing quantitive assessment of the effects of chest physiotherapy on patients subjected to I.P.P.V., is presented. The method involves measuring of the "alveolar" pressure using the end-expiratory retard mechanism available on the Bennet MA 1 (volume cycled) ventilator. Using this simple technique three different methods of physiotherapy were shown to significantly lower alveolar pressure, indicating an increase in pulmonary compliance. The inhalation of acetyl cysteine however, appeared to render treatment less effective in this regard.