The rate of local axial gas transport in oscillatory flow through a model of human central airways was measured to evaluate gas exchange during high-frequency oscillation. A rigid model of human central airways consists of asymmetrical bifurcations up to 3-5 generation, of which geometries were determined by the study of Horsfield et al. (J. Appl. Physiol., 31 (1971), 207.). A bolus of CO2 washout profiles. The rate of increase of effective diffusivity depends on the local flow conditions and differs with branches. The effective diffusivity in the left main bronchus is 3.2 times greater than that in the straight tube, whereas no significant difference is observed in the right main bronchus. In addition, the presence of a stationary period augments axial gas transport in intermittent oscillatory flow because it provides time for the occurrence of lateral mixing in the radial direction, i.e., axial diffusivity is 1.6 times greater on the average than that in sinusoidally oscillatory flow.