The fall in pulmonary compliance in mice with radiation pneumonitis is associated with increased microvascular leakage of plasma proteins into the alveolar spaces and increased surfactant phospholipids in the lung and alveolar fluid. In the present experiments we examined the effect of corticosteroid adminitration on these two effects and on pulmonary mechanics 16 wk after x irradiation of the thorax. Survival in irradiated animals that received corticosteroids was markedly better during the period of corticosteroid administration than that of irradiated animals that received no corticosteroids. The development of abnormalities in pulmonary mechanics and alveolar fluid surface tension appeared to be inhibited in the irradiated animals receiving corticosteroids as compared with irradiated animals not receiving corticosteroids. The increased microvascular protein leakage seen in the lungs of irradiated mice was not significantly different in the corticosteroid-treated group. However, corticosteroid adminstration was associated with a marked increase in the amount of phosphatidyl choline that could be recovered from the alveolar spaces by lavage, over and above the increase resulting from irradiation, and a significant increase in the incorporation of [14C]palmitate into phosphatidyl choline by lung slices. The beneficial effects of steroids in this variety of adult respiratory distress syndrome may be the result of augmented surfactant production which may contribute to the maintenance of relatively normal pulmonary mechanics despite substantial leakage of plasma proteins into the alveolar space.