Irradiation which acts directly and produces the reactive oxygen radicals by ionizing water molecules, causes significant morbidity and mortality. The muscle is damaged by direct action, oxygen radicals and the alterations of microcirculation and metabolism after irradiation. The changes of SOD immunoreactivities in muscles of the rats after irradiation were observed. The ultrastructural changes of the irradiated muscles with the pretreatment of SOD (superoxide dismutase) or without were also investigated. A total of 60 healthy Sprague-Dawley male rats weighing from 200g to 250g were used as experimental animals. Under urethane (1.15g/kg. IP.2 times) anesthesia,30 Gy irradiation to lower extremities by PICKER-C9 Cobalt-60 teletherapy unit was done. 15,000 unit/kg of SOD was administered intraperitoneally 1 hour before irradiation. The experimental animals were sacrificed 1 day, 3 days, 7 days, 2 weeks and 4 weeks after irradiation. The superficial portions of the mid-belly of the rectus femoris muscles were obtained and sliced into portions, 2 mm in length, 1 mm in width and in thickness. The specimens were prepared by routine methods for the electron microscopic observation. All preparations were stained with uranyl acetate and lead citrate and observed with a Hitachi-600 electron microscope. The other parts of mid-belly of the rictus femoris muscles were sectioned in 14 micrometer thickness with cryostat at -20 degrees C. The immunoreactivities of SOD by use of antihuman Cu, Zn-and Mn-SOD antibodies were observed. The results were obtained as follows . 1. After irradiation, the immunoreactivities of SOD in the rictus femoris muscle were decreased. 2 weeks after irradiation, the immunoreactivities of Cu, Zn-SOD were trace, which was lowest.4 weeks after irradiation, the immunoreactivities were trace or weak. 1 day after irradiation, the immunoreactivities of Mn-SOD were trace, which was lowest. The immunoreactivities of Mn-SOD were increased gradually 4 weeks after irradiation, the immunoreactivities of Mn- SOD were moderate or weak. 2. The ultrastructural changes in the rectus femoris muscles of the rats were getting severer and severer after irradiation. 2 weeks after irradiation, unclear A band and I band, myofibrillolysis, increased and dilated cistemae of sarcoplasmic reticulum and mitochondria with dilated cristae and electron lucent matrix were seen. 4 weeks after irradiation, lysis of sarcomere and increased cisternae of sarcoplasmic reticulum were seen. 3. The ultrastructural changes in the rectus femoris muscles of the rats were getting worse and worse after 3 days of irradiation with the pretreatment of SOD. 2 weeks after irradiation with the pretreatment of SOD, myofibrillolysis, increased and dilated cisternae of sarcoplasmic reticulum and damaged mitochondria were seen. 4 weeks after irradiation with the pretreatment of SOD, the ultrastructures of rectus femoris muscles were recovered to normal. Consequently, after irradiation of 30 Gy, the immunoreactivities of SOD are decreased and SOD attenuates the reversible changes of ultrastructures in muscles.