Abstract: Objective To investigate the protective effects of polarizing cardioplegia with Na⁺ channel inhibitor tetrodotoxin (TTX) in ex vivo rat heart preservation. Methods Using a Langendorff preparation, Wistar rat hearts were arrested and preserved with St.Thomas (STH) solution (n=10) or with polarizing cardioplegia (TTX, n=10) for 7 h in hypothermic storage (10 ℃). All the hearts then underwent 30 min of reperfusion. Pre-ischemia and post-ischemia indexes of the rat hearts were determined, including the hemodynamic parameters, myocardial enzymology, ATP content and ultrastructural changes. Results The recovery of hemodynamic parameters of the hearts in TTX group were better than those in STH group (P<0.01). In comparison with STH group, the leakage of creatine kinase (CK) and lactate dehydrogenase (LDH) in TTX group was significantly lower (P<0.05) and ATP level maintained a relative high level with better protected myocardial ultrastructure. Conclusion TTX polarizing cardioplegia provides more effective long-term hypothermic preservation of isolated rat hearts than STH.