A new liquid-liquid blood gas exchange system was investigated using a venovenous low flow extracorporeal circuit. A 2 m2 hollow fiber dialyzer served as the interface of the blood and oxygen carrier (a 38 percent FC-43, perfluorocarbon emulsion in a buffered electrolyte solution), which was continuously recycled through a bubble oxygenator. Experiments were performed on five mongrel dogs under general anesthesia. Upon the arrest of spontaneous ventilation, the dogs' lungs were inflated with 50% oxygen gas under a positive pressure of 10 cmH2O. After 10 min of apnea the dogs' PaO2 decreased to 37 +/- 14 mmHg, and the extracorporeal circulation was started at 10 ml/min/kg b./w. At 15 min the PaO2 had risen to 80 +/- 41 mmHg and at 30 min to 121 +/- 17 mmHg. The oxygen transfer was 8.3 +/- 2.3 ml/min. The extracorporeal circulation was continued 5h, when PaO2 reached 156 +/- 90 mmHg, and PaCO2 148 +/- 43 mmHg, then stopped. Fifteen minutes later, the PaO2 had returned to 32 +/- 10 mmHg. These findings indicate that our blood gas exchange system can supply a sufficient amount of oxygen to the body under apnea with continuous positive airway pressure.