Thirty-six mongrel dogs were ventilated with oxygenated fluorocarbon liquid (FX-80) for an hour and then reconverted to breathing gaseous oxygen. For several days after breathing this liquid, the dogs were hypoxemic while they breathed air. The authors attributed this to residual fluorocarbon in the lung and/or partial airway closure. By ten days after liquid ventilation, Pao2's had returned to pre-experimental control levels. Pathologic examination of the lungs three hours after termination of liquid ventilation disclosed an acute exudative inflammatory reaction largely confined to the bronchioles. By 72 hours the acute reaction had subsided and the dominant change consisted of vacuolated intra-alveolar macrophages, presumably containing fluorocarbon. At ten days, macrophages were still present, but generally in much smaller numbers; after a month only scattered small groups were found. At 18 months the lungs were normal.