In hope to elucidate possible changes in blood NP-SH levels when X-irradiation is made in single orfractionated dose, a whole body X-irradiation was done to rabbits either in single dose of 99r or in fractionateddose of 300r per day for three days. The NP-SH was measured at 1,3,5,24 and 48 post-irradiation hours, and theresults were compared with the normal value of the blood NP-SH. The results obtained are as follows; 1. The normalvalue of blood NP-SH in the rabbit was 2.11+−0.40 micron mol/ml. 2. In the single X-irradiation group, the bloodNP-SH decreased most prominently at five hours after-irradiation, and a tendency of recovery to the normal levelwas observed thereafter. 3. In the fractionated group, the blood NP-SH levels were higher than in the singleirradiation group throughout the experiment, and the levels were also higher than the normal in general.