Abstract: Objective:To verified the myocardial stunning phenomena caused by repeated exhaustive exercise on treadmills,and observed the changes of myocardial injury,electrocardiogram(ECG) and ultra structure of rats after exhaustive treadmill running.Methods:To select 22 SD class male rats,and are divided into 2 groups at random.A group is called control group and has 6 rats,B group is called training group and has 16 rats.According to normal or abnormal electrocardiogram after running,B group is divided B1 group called abnormal electrocardiogram group and B2 group called normal electrocardiogram group.The change of electrocardiogram on control,instant,10 min,20min,30min,1h,1.5h,2h,3h,6h,12h,24h is monitored.The area cellular necrosis in the parties anterior of left ventricle is analyzed on 24h after exercise and the histological changes of myocardial fiber are observed under Transmission Electron Microscope(TEM).Results:Electrocardiogram between pre and after exercise were not significance difference in B2 group,and in B1 group,J point had downward shift,T wave shifted down and appeared inversion partially,and appeared pathologic Q wave in a few of rats with running.At the end of the drove,the phenomenon is gradually the same to the before.The myocardial necrosis percent of Control group,B2 group and B1 group are respectively 17.93%±14.95%、29.05%±13.91%、21.34%±11.47%.There is no group difference each other.When the B1 of electrocardiogram was abnormal after running,under the electron-microscope,degeneration and swelling of mitochondria were found,but others had no significant change.and when 24h after running,under the electron-microscope,the degeneration and lyses of myofibrils,swelling of mitochondria vacuolar degeneration of carryon were found.Conclusions:Repeated exhaustive treadmill running can efficiently induce the phenomenon of the myocardial stunning and the exercises-induced myocardial stunning exists.There is certain relationship between the changes of electrocardiogram and micro-damage of mitochondria.