Changes in intrapleural pressure and air flow rate during a cough were recorded simultaneously with a high speed X-ray motion picture of the chest which permitted visualization of the trachea. Analysis of these records showed that the changes in diameter of the trachea were synchronous with changes in intrapleural pressure, once the glottis had opened and expiratory flow had started. Analysis of several X-ray motion pictures of the bronchial tree yielded no evidence of a wave-like progression of diameter changes along the bronchial tree. Effect of transmural pressure gradients upon the cross section area of the lumen of an isolated human trachea was investigated. It was found that a transmural pressure of 40 cm H2O reduced the cross section area to about 1/5 its original value.