The study was undertaken to evaluate the radioprotective effect of pitressin. A total of 125 mice were usedfor the experiment, of which 10 mice we used to assess pharmacological effect of pitressin, 40 mice to assessdifference of radiation effect on single and fractionated whole body irradiation as contral group, and 75 mice toassess radioprotective effect of pitressin accoridng to single and fractionated whole body irraadiation. Wholdbody inrradiation was done was done 1,200 rads on single dose groups and first 600 rads and second 600 rads onspit dose groups with one week interval using 6MeV linear accelerator. Pitressin administration was done byintraperitoneal route with dosage of 2.0 unit and 6.0 unit on each groups. Evaluation was pointed 7 day survivalrate under the concept of acute intestinal death after whole body irradiation around the dose of LD 50. Finally,the gross and microscopic change of intestine was observed on dead mice. The results obtained are summarized asfollows; 1. The Ld50/7 of mice obtained in this experiment was 600 rads. 2. The seven day servival rate infractionated irradiation groups was singificantly higher than single irradiation groups, so that it is to say thatfractionated irradiation decreases radiobiologic damage to the animal. 3. The seven day survival rate of thepitressin treated group was significantly higher than that of the control group. 4. Radioprotective effect ofpitressin was increased in proportion to the amount. In summary, pitressin has singificant radioprotective effecton whole body irradiation in proportion to the amount and either on single and fractionated irradiation, thus thisresult would be helpful to further clinical study of radioprotection.