The effects of radiation and Cis dichlorodiammineplatinum(II) (Cis-DDP) were assessed in rectum of rats by histopathological changes. Rats were exposed to single doses of X ray(6~10 Gy) without or with Cis-DDP(2.5mg/kg). In combined group, Cis-DDP was given 30 minntes before or immediately after irradiation. Cis-DDP alone showed inflammatory cell infiltration and increased goblet cells in the mucosa and edema and fibrosis of submucosa with vascular sclerosis. With increased radiation dosage. such changes were aggravated. Necrosis of muscle layer developed 8 Gy irradiation. In groups with combination treatment of X-ray and Cis-DDP. changes of mucosa and submucosa were not significantly different from radiation alone group, but necrosis of the muscle layer was developed in 6 Gy combination group and degree was more severe in 8 and 10 Gy combination group compared to radiation alone group. There was no difference according to the timing of Cis DDP administration before or after irradiation. This result suggests Cis DDP enhance the radiation effect on the rectum of rats and enhancement ratio was 1.3 as the endpoint was necrosis of the muscle layer.