Most of chronic hemodialysis patients are combined with cardiopathies, such as ischemic heart disease, heart failure, and arrhythmia. Heart rate variability as an index of cardiac autonomic activity is falling in many hemodialysis patients, which can have a concern with the arrhythmogenicity. In addition, QT dispersion as an index of an electrical variability in the ventricular repolarization process increases by hemodialysis, which may tightly relate to the occurrence of ventricular arrhythmia. On the other hand, tendency of the magnesium accumulation is observed in renal insufficiency patients. In our study, plasma concentration of ionized magnesium showed a positive correlation with pNN50 and HF, indicating that this ion may enhance the parasympathetic activity resulting in a cardioprotection. Plasma ionized magnesium also showed a negative correlation with QT dispersion, suggesting that magnesium plays a role of maintaining myocardial electrical stability in hemodialysis patients.