Abstract: The effects of ^#64 iodium-heterocycle(3,6-di-dimethylamino-dibenzopyriodonium formate, IHC-64) on the thresholds of excitation and fibrillation of the ischemic rabbit myocardium were investigated. The threshold of excitation and fibrillation was determined as the minimum voltage (MV) required to drive the heart for at least five beats or to induce ventricular fibrillation (VF) separately. Ⅳ IHC-64 2～4 mg/kg increased markedly MV to induce VF (p<0.01), and showed no effect on the excitability. These results suggest that IHC-64 should be of benefit when used in myocardial ischemia.