Combined high frequency oscillation (HFO) with conventional ventilation was used on a group of 12 ventilator dependent neonates in order to investigate the frequency response of the respiratory system to HFO. Pressure oscillations were measured at the airway opening with a transducer and volume oscillations at the chest wall using a calibrated respiratory jacket. Pressure oscillations increased with increasing frequency but the oscillating volumes showed a variable pattern with maximum oscillating volumes of up to 4 ml/kg. Three babies showed no change in volume between 2-25 Hz, 3 babies showed increasing volumes up to 25 Hz and the remainder showed a maximum volume (or resonant frequency) at 15-20 Hz. Oscillating volumes were higher when the conventional ventilator was in the expiratory phase. The differences in frequency response were unrelated to severity of lung disease, birth weight, age at testing, or size of the endotracheal (ET) tube. Local factors in the ET tube and large airways may account for some of the observed differences.