Continuous-flow apneic ventilation (CFAV) by endobronchial insufflation of conditioned gas was evaluated in dogs during thoracotomy. In Group 1 (n = 6), dogs were anesthetized with pentobarbital (25 mg/kg). An endobronchial catheter (2.5 mm ID) was introduced into each mainstem bronchus using a fiberoptic bronchoscope and held in place by an endotracheal tube. Before the onset of CFAV (total flow 1.0 1 · kg-1· min-1, the animals were paralyzed with pan-curonium bromide and muscle relaxation was monitored with a peripheral nerve stimulator. The CFAV delivery system consisted of a flow meter, air/oxygen blender, oxygen analyzer, heated humidifier, and ultrasonic spirometer. Blood gas values were measured after 30 min of spontaneous ventilation, and CFAV with: 1) closed chest, fractional inspired O2concentration (FIO2) 0.21; 2) open chest, FIO20.21; 3) open chest, FIO20.21, continuous positive airway pressure (CPAP) 5 mmHg; and 4) open chest FIO20.4, CPAP 5 mmHg. This last combination resulted in a mean PaO2of 113.1 ± 5.5 (SEM) mmHg and a PaCO2of 35.0 ± 2.1 (SEM) mmHg. In Group 2 (n = 6), animals with open chests were ventilated with CFAV (FIO20.4 and CPAP 5 mmHg) for 5 h. Adequate oxygenation and ventilation were achieved. PaCO2after 5 h of CFAV was 41.8 ± 1.9 (SEM) mmHg compared with 40.8 ± 1.9 (SEM) mmHg during spontaneous breathing. PaO2after 5 h of CFAV was 138.1 ± 11.7 (SEM) mmHg. There were no significant changes observed in vascular pressures. Significant differences in other hemodynamic parameters were probably due to pentobarbital anesthesia. Adequate gas exchange can be achieved during CFAV in dogs with open chests for 5 h.