In the previous work, it has been shown that the reaction of blood platelet aggregation is risen during the early days following ,γ, ray irradiation of large doses. In this paper, it is reported that changes of Rat PGI,2,-like activity with inhibiting platelet aggregation after ,γ, ray irradiation of 8 Gy. The results indicated that the activity was reduced at the first day after radiation, then recovered at the third day, lowered again at the 5-7 days. The changs have significant concern between the increase of blood platelet aggregation of Rats and the inhibition of PGI,2,-like activity of blood vessel wall following ,r, ray irradiation.