Prolonged exposure to hyperoxia in order to compensate for inefficient ventilation can lead to progressive pulmonary damage and death. Since pulmonary macrophages control bacterial and viral penetration, we studied the effect of hyperoxia on pulmonary alveolar macrophages from newborn and adult rats, kept in air or in a 95% normobaric oxygen atmosphere. The viability of pulmonary alveolar macrophages in bronchoalveolar lavages of newborn rats after 3 d in oxygen was significantly lower than that of newborn rats after 3 days in air. The functional capacity, as measured by the phagocytosis of Candida was similarly affected and these observations mays explain why infections develop. Adult rat macrophages were not sensitive to hyperoxia.