The Study was undertaken to evaluate the radioprotective effect of pitressin. A total of 125 mice were used for the experiment, of which 10 mice were used to assess pharmacological effect of pitressin, 40 mice to assess difference of radiation effect on single and fractionated whole body irradiation as contral group, and 75 mice to assess radioprotective effect of pitressin according to single and fractionated whole body irradiation. Whole body irradiation was done 1, 200 rads on single dose groups and first 600 rads and second 600 rads on split dose groups with dosage of 2.0 unit and 6.0 unit on each gropus. Evaluation was pointed 7 day survival rate under the concept of acute intestinal death after whole body irradiation around the dose of LD50. Finally, the gross and microscopic change of intestine was observed on dead mice. The results obtained are summarized as follows : 1. The Ld50/7 of mice obtained in this experiment was 600 rads. 2. The seven day survival rate in fractionated irradiation groups was significantly higher than single irradiation groups, so that it is say that fractionated irradiation decreases radiobiologic damage to the animal. 3. The seven day survival rate of the pitressin treated group was significantly higher than that of the control group. 4. Radioprotective effect of pitressin was increased in proportion to the amount. In summary, pitressin has significant radioprotective effect on whole body irradiation in proportion to the amount and either on single and fractionated irration, thus this result would be helpful to further clinical study of radioprotection.