The objective of this work is to develop understanding of the local fluid dynamic mechanisms that underpin gas exchange in high-frequency oscillatory ventilation (HFOV). The flow field during HFOV was investigated experimentally using particle image velocimetry in idealized and realistic models of a single bifurcation. Results show that inspiratory and expiratory fluid streams coexist in the airway at flow reversal, and mixing between them is enhanced by secondary flow and by vortices associated with shear layers. Unsteady flow separation and recirculation occurs in both geometries. The magnitude of secondary flow is greater in the realistic model than in the idealized model, and the structure of secondary flow is quite different. However, other flow structures are qualitatively similar.