Abstract: AIM: To study the dynamic changes of serum cTnI and myocardium histomorphology in different time periods after exhaustive exercise in rats in order to evaluate delayed-onset myocardial damage during recovery phase after exercise. METHODS: Animal models of exercise-induced myocardial injury were established according to Thomas’s method. Healthy untrained 87 male Wistar rats were selected and randomly divided into one sedentary control group (n=10) and seven exhaustive exercise groups according to times of recovery phase after exercise (immediately after, 3, 6, 12, 24, 48 and 96-h groups with 11 rats in each group). Blood samples were obtained and cTnI in serum was measured. Changes of myocardial histopathology were examined immediately after, 3, 6, 12, 24, 48 and 96 h after exercise. RESULTS: Compared with that in sedentary control group, the level of serum cTnI was significantly increased at immediately after, 6 and 48 h after repeated exhaustive exercise (P<0.05 and P<0.01), peaked at 6 h and 48 h, maintained the higher level at 3, 12 and 24 h and basically recovered at 96 h post-exercise. After 1 week consecutive daily exhaustive exercise, myocardial injuries of different degrees were observed at immediately after, 3, 6, 12, 24, 48 and 96 h after exhaustive exercise under light microscope. Myocardial injury was most severe at 48 h and was alleviated at 96 h post-exercise. CONCLUSION: After long-term excessive exercise and/or exhaustive exercise, the pathological changes in myocardium tissue occurred in a time-dependent manner. Similar to skeletal muscle, delayed-onset myocardial injury is also induced by exhaustive exercise.