Gas transport during high-frequency oscillation was investigated in vitro using CO2 elimination from the lung surrogate as a measure of gas transport efficiency. The length of the connecting tube between the piston pump and the three-port connector did not affect gas transport efficiency if the oscillatory volume (VDEL) was constant; inserting an additional tube between the three-port connector and the endotracheal tube decreased gas transport efficiency dramatically. In contradistinction, increasing VDEL caused a steep rise in gas transport efficiency as soon as VDEL surpassed the volume of the tubes connecting the lung surrogate with its surroundings. As gas transport efficiency was found to be very sensitive to the net oscillatory volume, i.e. VDEL minus the volume of the tubes connecting the lung and the surroundings, direct wash-out was considered to be an effective gas transport mechanism during high frequency oscillation. Two preliminary experiments on dogs allowed us to substantiate this hypothesis in vivo.