Ten anesthetized, paralyzed adult cats were ventilated by high frequency chest wall compression (HFCWC) at 3, 5, 7, and 9 Hz by means of a single chamber cuff enclosing the thorax from the axillae to the xyphisternum. The effects of HFCWC in terms of gas exchange, end-expiratory lung volume, and respiratory system compliance were compared to conventional intermittent positive pressure ventilation (IPPV) (30 breaths/mn). HFCWC and IPPV were compared at three levels of matched end-expiratory airway pressure [continuous positive airway pressure (CPAP)/positive end-expiratory pressure/( PEEP) of 0, 2, and 5 cm H2O]. In the absence of CPAP, HFCWC resulted in a marked decrease (up to 50%) in end-expiratory lung volume with significantly lower PaO2, lower compliance, and higher alveolar-arterial oxygen gradient than during IPPV. No differences in PaO2and a-ADO2were observed when HFCWC was combined to CPAP ≥2 cm H2O. At frequencies below 9 Hz, PaCO2became significantly lower during HFCWC + CPAP than during IPPV. During HFCWC + 2 cm H2O CPAP, lung volume was lower than during IPPV + 2 cm H2O and similar to the volumes observed during IPPV + 0 positive end-expiratory pressure. Additional studies in six cats at HFCWC + 3 cm H2O confirmed that CPAP>2 cm H2O more than adequately compensated the decrease in lung volume associated with HFCWC alone. Peak cuff pressures between 14 and 17 cm H2O generated oscillary tidal volumes between 4.5 and 2.1 ml/kg. The size of the oscillatory volume was significantly affected by increasing frequencies (decrease in tidal volume) and increasing levels of positive airway pressure (increase in tidal volume). We conclude that in cats with normal lungs, HFCWC can provide for normal gas exchange, provided that it is combined with low level CPAP in order to prevent the occurrence of airway closure associated with HFCWC alone.