Twenty-four dogs underwent in vivo left pulmonary hilar occlusion with the lung continuously expanded at 10 centimeters or 25 centimeters of water pressure to determine the period of pulmonary ischemia that may be tolerated before consistent pulmonary edema and congestion develop after lung revascularization. Consistent and prolonged pulmonary edema and congestion that caused death of the dog occurred in at least one-half of the dogs when the period of hilar occlusion was extended beyond six hours. Elevation of the left pulmonary artery pressure was only a rough measurement of the severity of the anoxic pulmonary injury. Expansion of the lung at 10 centimeters of water continuous pressure was more beneficial than was expansion at 25 centimeters of pressure. Cyclic ventilation with slight negative-expiratory pressure provided less support to the lung than did continuous expansion at either pressure tested. Intial decreases in both ventilation and perfusion isotope uptake and the percentage of the total volume of oxygen uptake per minute by the ischemic lung returned to near normal levels in three weeks in dogs that survived. Lung expansion during periods of ischemia appears to prevent alveolar collapse and to facilitate oxygenation.