Abstract: The changes of monophasic action potential (MAP) in ischemic and reperfused feline ventricle in siiu were studied. When the feline coronary artery was ligated the MAPD₅₀ was found to be markedly shortened, the T wave and S-T segment were elevated evidently. Within seconds of releasing the coronary occlusion ventricular fibrillation and death occurred in all animals. Intravenous injection of BTHP markedly prolonged the MAPD₅₀ and MAPD₉₀ and reduced the heart rate, while no influence on T wave and S-T segment were observed. After occlusion of the coronary artery, the T wave and S-T segment only increased slightly and the MAPD₅₀ recovered to its control level. During reperfusion the occurrence of ventricular fibrillation was significantly reduced. In contrast, prazosin, verapamil and propranolol did not show significant effect on changes of MAP, but BTHP and Pra, Ver and Pro all showed protective effect on feline isohemic and reperfused myocardia by ligation and reperfusion of the coronary (?)rtery with decrease of the number of arrhythmia, ventricular fibrillation and death. The order of antiarrhythmic action of these agents was BTHP=Pra>Ver>Pro. The total incidence of ventricular fibrillation reduced from the control values of 100% in ischemia/reperfusion heart to 0% (BTHP and Pra),20% (Ver) and 40% (Pro).