The effects Cis-dichlorodiammineplatinum (II) (Cis-DDP) on the irradiated lung were assessed by histopathologic changes. In radiation alone group, right lung of rats were exposed to X-ray 10, 15, 20, 30, 40 Gy in a single dose and in combined group, Cis-DDP(6mg/kg) was administered immediatelly after irradiation of same dose of X-ray of radiation alone group. Histopathologic examination was done after 4 weeks after experiments. The early histopathologic changes of lung by irradiation was patchy infiltrations of macrophage in the alveolar space, desquamation of the alveolar septae and perivascular inflammatory cell infiltrations which was appeared in 10 Gy irradiated group and was more aggravated with incressing radiation dose. The destruction of alveolar septae was noted in 20 Gy irradiation group and it was severer in 30 Gy and 40 Gy group. In combined group of radiation and Cis-DDP, the destruction of alveolar septae was appeared in 15 Gy group and the changes of alveolar space such as edema ana hemorrhage was diffuse and severer than radiation alone group. Degenerative changes of vascular endothelial cells and alveolar epithelial cells with type II pneumocytes proliferation were more prominent in combined group than radiation alone group in electron microscopic findings. This result suggests Cis-DDP enhance the radiation pneumonitis of rat lung and enhancement ratio was 1.3 as the endpoint was the destruction of the alveolar septae.