Morphologic changes of radiation pneumonitis in the rat included atrophy and hyalinization. Microscopic alterations consisted of depopulation of alveolar lining surfaces, sparse accumulation of macrophages within alveoli, and hyalinization of alveolar walls. The reaction was slowly progressive, developed in the absence of exudative inflammation, and was not modified by cortisone or oxytetracycline. A suggested pathogenesis involves mitotic block in the alveolar cells which detach and, depending on the patency of the macrophage outflow tract, either accumulate in the air sacs or are transported out of the lung. This concept does not require the process of inflammation, and it is consistent with a temporal account of the changes observed.