Chickens exposed to hyperbaric oxygen (3 to 5 ATA) or to 100% oxygen at atmospheric pressure for prolonged periods showed no pathologic pulmonary damage, although hydrogen peroxide did accumulate in their airways. Superoxide dismutase in the peripheral nucleated red blood cells and the rate of oxidation of specifically labeled glucose were increased in chickens exposed to hyperbaric oxygen for three hours. Superoxide dismutase increased in the lung lavage of the rabbits and in the peripheral nucleated red blood cells. Glucose oxidation was unchanged. The difference in superoxide dismutase is probably the result of the desquamated, damaged cells found in the rabbit lung lavage. That only rabbit bronchi are lined with oxidant-sensitive ciliated cells suggests that the absence of these ciliated cells in fowl may well be responsible for the observed resistance of these birds to oxidant damage by oxygen.