The study was undertaken to evaluate the radiological and microscopic findings of radiation penumonitis. All 50 rats, which used for the experiment, received a single dose of 2,000 rads of radiation on right lung using 60 Co teletherapy. Chest teleroentgenograms were checked weekly for 13 weeks and microscopic examinations of irradiated lung were performed monthly after irradiation. The results obtained are summerized as follows. 1. In roentgenologic examination, radiation pneumonitis began to appear 4 weeks after irradiation and progressed with time intervals and began to resolve after 11 weeks. 2. Chest roentgenologic findings were pneumonic consolidation, atelectasis, and pleural effusion of irradiated right lung with compensatory hyper-inflation of left lung. 3. In the microscopic examination, pneumonitis appeared 2 weeks after irradiation and progressed with time intervals and began resolve after 8 weeks. 4. The microscopic findings were fibrin deposit in alveoli and in reased septal cellularity, vascular thickening , edema and congestion. After septal fibrosis and hyperplasia of bronchial mucosa, suggesting early regenerative change of radiation fibrosis, were noted 12 weeks after irradiation.