Breathing pure oxygen at 1 atmosphere pressure causes significant damage to respiratory tissue of monkeys. Alveolar lining epithelium is almost completely destroyed after 4 days. The destroyed type 1 epithelium is replaced after 7 days of exposure by a cellular lining consisting exclusively of type 2 epithelial cells. The air-blood tissue barrier becomes progressively thickened, particularly in the phase of proliferative reaction which appears after 7 days of exposure to pure oxygen. In the early exudative phase the slight increase in the thickness of the barrier is caused by swelling of the endothelial cells and by interstitial edema. The endothelium seems to constitute the target tissue in the early reaction to oxygen toxicity. After 7 days of exposure the destruction of endothelial cells produces a 50 per cent decrease in their total volume; on the 12th day this volume is one-third that of the control values. The finding of interstitial hemorrhage and other morphologic manifestations indicates a capillary destruction which has been clearly demonstrated rat lungs exposed to pure oxygen. However, probably because of dilation of persisting vessels and opening of so-called" dormant capillaries" neither the capillary volume nor the surface area changes. This suggests that the longer survival of monkeys exposed to oxygen allows a remodeling and redistribution of alveolar vasculature. Recovery in room air for 56 and 84 days, preceded by 8 and 13 days of exposure, respectively, allows substantial structural recuperation. At this time the diffusing capacity of the air-blood barrier, as calculated from the morphometric data is practically normal.