A comprhensive morphologic study of pulmonary oxygen toxicity was undertaken on 78 newborn mice of a single, highly inbred strain continuously exposed to 100 per cent oxygen at normal atmospheric pressure for 7 days. The survival rate was 95 per cent through the first 4 days and 75 per cent through the 7th day. Our findings indicate that response to continuous exposure to 100 per cent oxygen can be divided arbitrarily during the first 7 days into three different phases. During the first, or acute toxic, phase (which generally extends into the 3rd day of exposure), mechanisms, probably adaptive in nature, are sufficiently developed in some of the newborn animals to permit them to survive a continuous 7-day exposure. The acute toxic phase is followed by the critical, high mortality phase, which lasts 3 to 4 days. During this phase, which can be subdivided into a continuum of several stages, the changes seen in the initial, acute toxic phase may progress to more pronouced injury. If the injury is severe, massive pulmonary edema and hemorrhage occur, accounting probably for the deaths of most of the animals in previous similar experimental studies and of 25 per cent of our animals by the 7th day. The second stage of this high mortality phase, being more clearly reactive, consists of a progression of reactive mitochondrial changes, proliferation of granular pneumocytes, and a return to normal of the features of the membranous pneumocytes. The beginning of the third phase, the chronic injury and repair phase, is characterized by the appearance of fibroblasts and a concomitant moderate increase in the amount of collagen.