It is well recognized that the intrapulmonary currents of air have an important bearing on bronchial obstruction. Obstruction from accumulation of secretions complicates bronchitis most frequently when the breathing is superficial. Both prevention and relief of obstruction may be promoted by the application of deep breathing exercises and of vigorous, well controlled coughing. Following the extensive investigations of Henderson and Haggard in the production of hyperpnea by the inhalation of carbon dioxide, the practice of administering this gas to patients after operation has become a routine measure in many surgical clinics. Nevertheless, the aerodynamics of bronchial obstruction is not understood in certain essential respects. It is not known, for instance, how deep breathing introduces air into a collapsed section of lung, the bronchi of which are filled with mucus; nor, under the same conditions, how cough evacuates the mucus. Conversely, the cause is not clear for the failure of deep