Turbulence during pulsatile flow has been suggested as a possible mechanism to enhance the transport of gases during high-frequency ventilation. Experimental studies on oscillatory flow in straight, circular tubes have identified three types of flow: (a) laminar; (b) conditionally turbulent, in which high-frequency disturbances occur during the decelerating phase of the flow cycle but relaminarize by the beginning of the subsequent accelerating phase; and (c) fully turbulent flow, in which disturbances occur throughout the flow cycle. Fully turbulent flow has been observed only when a mean flow is present, and only laminar or conditionally turbulent flow has been observed for purely oscillatory flow. A critical Reynolds number based on the Stokes layer can be defined, and transition Reynolds numbers between 400 and 550 have been experimentally determined for purely oscillatory flow in a circular tube, although lower values are expected for physiological flows. There are some indications that the structure of oscillating turbulent flow is similar to steady turbulent flow, and preliminary work in our laboratory shows that the spectral content of flows during high-frequency ventilation is similar to that in steady turbulent flow.