We have measured tidal (VT), entrained (Ve) and "blowback" (Vbb) volumes during high frequency jet ventilation (HFJV) through a Mallinckrodt Hi-Lo Jet tracheal tube in anaesthetized patients. The above volumes were calculated by digital integration of the appropriate regions of flow curves derived from a pneumotachograph placed between the bias flow tubing and the tracheal tube. At a driving pressure of 1 bar, lung minute ventilation increased with increasing ventilatory frequency, whilst tidal volumes decreased. The contribution of entrainment to tidal volume (Ve/VT) remained constant, although the volumes entrained were relatively small and varied widely from subject to subject. Blowback volumes were considerable, especially at ventilatory frequencies used clinically (1-2 Hz). We conclude that it is not possible to entrain predictable concentrations of volatile agents from the low pressure bias flow during HFJV.