In unanesthetized cats, extrathoracic mechanisms appear to play a more important part in regulating the respiratory cycle than has been appreciated previously. From the efferent standpoint, the larynx provides mechanically important, variable regulation of expiratory airflow, preventing rapid expiratory collapse of the respiratory system during quiet breathing, but allowing rapid flow rates to be achieved during hyperpnea. On the sensory side, the recent discovery of large numbers of slowly adapting stretch receptors in the extrathoracic trachea of dogs is of considerable interest; the physiological role of these receptors has not yet been worked out, but indirect evidence suggests that they may participate in the reflex regulation of expiratory airflow. In the proposed studies, the applicability of the role of the larynx, as demonstrated in cats, will be evaluated in human subjects by using a fiberoptic bronchoscope to monitor vocal cord movements during quiet breathing and hyperpnea. This system will also be used to assess the response to single breath changes in expiratory resistance. The extent to which stretch receptors in the extrathoracic trachea may influence the regulation of expiratory airflow will be directly tested in unanesthetized, unrestrained cats.