In hope to elucidate possible changes in blood NP-SH levels when X-irradiation is made in single or fractionated dose, a whole body X-irradiationi was done to rabbits either in single dose of 900r or in fractionated dose of 300r per day for three days. The NP-SH was measured at 1,3,5,24 and 48 post-irradiation hours, and the results were compared with the normal value of the blood NP-SH. RESULTS : 1. The normal value of blood NP-SH in the rabbit was 2.11$pm$0.40 $mu$㏖/ml. 2. In the single X-irradiation group, the blood NP-SH decreased most prominently at five hours after-irradiation, and a tendency of recovery to the normal level was observed thereafter. 3. In the fractionated group, the blood NP-SH levels were higher than in the single irradiation group throughout the experiment, and the levels were also higher than the normal in general.