The overall goal of our research is to develop an understanding of the mechanisms by which inspired air undergoes mixing in the human pulmonary airways. This study is subdivided into three projects: Project I using an inert gas bolus-response technique, we are examining, in human subjects, the effect of separate variations of inspiratory and expiratory flow rate upon the longitudinal mixing of sulfur hexafluoride as compared to helium. Project II Using a physical model of the central and upper airways, we will determine the effect of local turbulence generated by the larynz during inspiration upon the longitudinal mixing in the central airways. Project III The rates of radial mixing of thermal energy and water vapor and the associated airway wall temperatures and humidities are being inferred from measurements of single-breath expired temperature profiles.