Abstract: This study was designed to monitor continuously the changes of electrocardiogram（ECG）in early stage of myocardial infarction in dogs.17 mongrel dogs were subjected to left anterior descending coronary artery（LAD）ligation about 1.5 cm from its origin,88.9% of which got ventricular fibrillation（VF）starting from 1.5 to 35 minutes after ligation,with an avdrage of 15 minutes.In general,this kind of ischaemic VF did not stop by oneself.Injured pictures of superacute period in ECG were found immediatly after LAD ligatiion.These ECG changes of superacute period were obvious but continued for a rather short time.Cardiac electric alternating phenominan,ventricular tachycardia with pleomorphic and polyoriginal QRS waves and torsional ventricular tachycardia with widened QRS waves and prolonged Q-T interval,often give a warning signal for developing VF.