Objectives Recent studies have described regional differences in the electrophysiology and pharmacology of ventricular myocardium in canine, feline, rat, guinea pig, and human hearts. This has been shown to be due to a smaller IKs and a lager sodium-calcium exchange current (INa-Ca) and late INa in M region (deep subepicardial to midmyocardial). Studies from our laboratory have found a new repolarization current-nonselective cation current (NSCCs) existing in rabbit right ventricular myocytes. Methods We examined the characteristics of NSCCs in epicardial, M region, and endocardial cells isolated from the rabbit left ventricle with standard microelectrode and whole-cell patch-clamp techniques. The permeability to Na+, K+, Li+, Cs+ but not to Cl- indicating that it was a nonselective cation current. Gd3+ (0.1 mmol/l) and La3+ (0.1 mmol/l) can block the current markedly. Results Further characterization of NSCCs was significantly smaller in M cells than in epicardial and endocardial cells. NSCCs current density was significantly smaller in M cells than in epicardial and endocardial cells. With repolarization to -80 mV, INs current density was (-0.44±0.05) PA/PF in endocardial cells, (-0.12±0.05) PA/PF in M cells and (-0.28±0.07) PA/PF in epicardial cells; and with repolarization to +30 mV, INs current density was (1.09±0.29) PA/PF in endocardial cells, (0.38±0.09) PA/PF in M cells and (0.91±0.32) PA/PF in epicardial cells. Conclusions Transmural dispersion of repolarization was due to the heterogeneity of NSCCs in rabbit left ventricle epicardial, endocardial myocytes and M cells. These findings may advance our understanding of the ionic basis for our understanding of factors contributing to the development of cardiac arrhythmias. Objectives Recent studies have described regional differences in the electrophysiology and pharmacology of ventricular myocardium in canine, feline, rat, guinea pig, and human hearts. This has been shown to be due to a smaller IKs and a lager sodium-calcium exchange current (INa Studies from our laboratory have found a new repolarization current-nonselective cation current (NSCCs) existing in rabbit right ventricular myocytes. Methods We examined the characteristics of NSCCs in epicardial, M region, and endocardial cells isolated from the rabbit left ventricle with standard microelectrode and whole-cell patch-clamp techniques. The permeability to Na +, K +, Li +, Cs + but not to Cl- indicating that it was nonselective cation current. Gd3 + ( 0.1 mmol / l) and La3 + (0.1 mmol / l) can block the current markedly. Results Further characterization of NSCCs was significantly smaller in M ​​cells than in epicardial and endocardial cells s. NSCs current density was significantly smaller in M ​​cells than in epicardial and endocardial cells. With repolarization to -80 mV, INs current density was (-0.44 ± 0.05) PA / PF in endocardial cells, (-0.12 ± 0.05) PA / (-0.28 ± 0.07) PA / PF in epicardial cells; and with repolarization to +30 mV, INs current density was (1.09 ± 0.29) PA / PF in endocardial cells, (0.38 ± 0.09) PA / PF in M cells and (0.91 ± 0.32) PA / PF in epicardial cells. Conclusions Transmural dispersion of repolarization was due to the heterogeneity of NSCCs in rabbit left ventricle epicardial, endocardial myocytes and M cells. These findings may advance our understanding of the ionic basis for our understanding of factors contributing to the development of cardiac arrhythmias.