Muscle damage in the legs of male Wistar rats after exposure to high-dose radiation was evaluated by observations of biopsied tissue made with light and transmission electron microscopy. Both legs of about 40 mature male Wistar rats were exposed to 80 Gy 6 MV X rays from a linear accelerator given in 40 fractions of 2 Gy. At intervals starting from the first day to 12 months after irradiation, serial muscle biopsies were obtained by sacrificing the rats on different days according to the experimental design. The irradiated gastrocnemius muscle displayed a progressive structural change that began immediately after the completion of the exposures. Morphological changes in irradiated muscles were characterized by hemorrhage, lymphocytic infiltration and obvious vascular destruction. Increased amounts of collagen between the muscle fibers were present even at 12 months after irradiation. There were significant changes in the number, size and structure of mitochondria in the muscle cells from the first day until 1 year after irradiation. Even 12 months after the final exposure, no recovery of muscle tissue was seen by light and transmission electron microscopy.