We examined changes in the blood pressure and the superoxide dismutase (SOD) activities in aortic tissue after gamma-irradiation to the chest regions of spontaneous hypertensive rats (SHR) and Wistar Kyoto rats (WKY), and obtained the following results. After 5 Gy irradiation, there was no change with time in the blood pressure in WKY rats, while a transient decrease (3 days later) was observed in SHR rats. The Cu/Zn-SOD activity in the aorta of SHR was less than that of WKY. 5 Gy irradiation induced an increase in the Cu/Zn-SOD activity in SHR nearly to the level observed in WKY, which remained unchanged upon irradiation. Therefore, unlike high-dose irradiation, irradiation at a relatively low-dose of 5 Gy to SHR appears to increase the aortic Cu/Zn-SOD activity, which is lacking in SHR, leading to a decrease in the blood pressure.