PURPOSE: We examined the time course change of the capillary structure in rat skeletal muscle one, three and seven days after strenuous eccentric exercise. METHODS: The right gastrocnemius muscles of anesthetized male wister rats (n = 20) were subjected to 300 (60 times, 5 sets) controlled eccentric contractions induced by electrical stimulation of surface electrodes. The contralateral gastrocnemius muscle was used as a normal control. All morphometric parameters with respect to capillary and muscle fiber were determined in in situ perfused gastrocnemius muscles in red (Gr; predominant in slow-twitch fiber) and white (Gw; predominant in fast-twitch fiber) portions. RESULTS: Damaged muscle fibers (infiltration of mononuclear cell or a swollen appearance) at days one, three and seven were observed in both Gr (29.3 to 53.9% damaged fibers) and Gw (58.9 to 86.8% damaged fibers) of exercised legs. We could not observe collapsed or obstructe capillaries in any exercised legs in light and electron micrographs of transverse sections. Furthermore, capillary endothelial cells had a normal structure in exercised legs. These results indicate that capillaries were not lost one to seven days after strenuous eccentric exercise. On the first and third day, however, capillary tortuosity and luminal diameter changed in exercised legs. The ratio between minimal and maximal capillary diameter in a transverse section (i.e., capillary tortuosity) significantly differed from control (Gr: 0.75 ± 0.02, Gw: 0.79 ± 0.03) and exercised legs (Gr: 0.65 ± 0.03, Gw: 0.66 ± 0.04) one day after exercise. The mean capillary luminal diameter was significantly increased in exercised legs after one day (Gw: +11.5%) and three days (Gr: + 14.9%, Gw: +25.2%) compared with control legs. These morphological changes in capillaries might be related to muscle fiber repair through the changes in hemodynamics such as erythrocyte and leukocyte flow. CONCLUSION: We conclude that 1) capillary endothelial cell structure was rather stable against strenuous eccentric exercise compared with the muscle fiber structure, and 2) changes in capillary tortuosity and distensibility occur in the degenerated muscle up to three days after the exercise period.