The aim of this study was to investigate the effects on muscle tissues of increasing amounts of exercise under normal and high temperatures. An experimental swimming test was applied to completely exercised rats. There were 42 adult male rats divided into 4 groups, including normal control rats, rats which swam to death under room or high temperature conditions, and rats swimming under high temperatures. Immediately after the exercise, the soleus (SOL) and extensor digitorum longus (EDL) muscles were removed by surgical excision. All specimens were frozen in isopentane cooled to -70 degrees C in liquid nitrogen. Cryostat cross sections were cut and stained using histologic and histochemical staining methods to observe pathological changes. The major changes included interstitial edema, muscle fiber atrophy, muscle fiber necrosis, hypercontracted fibers, and ragged red fibers. The rats exercised to death under high temperatures had more apparent changes than those not exercised to death. These results indicate that pathological changes in muscle will be brought about by increasing the amount of exercise and environmental temperature. Since there are more fast twitch fibers in the EDL than in the soleus, pathological changes in muscle tissues are more severe in the EDL.