Electrophysiological studies were performed to evaluate the effects of MgSO4 on human conduction system. Nine patients who received 20 mg/kg of MgSO4 did not show any significant changes in the electrophysiological parameters except for prolongations in the PR and AH intervals. After the infusion of 40 mg/kg of MgSO4 in eleven patients, sinus cycle length and corrected sinus node recovery time were increased but statistically insignificantly. Sinoatrial conduction time, the PR interval and AH interval were increased. The effective refractory period (ERP) of right atrium, ERP of atrioventricular node (AV node), ERP of right ventricule and ERP/QTc were increased significantly. These findings suggests that the antiarrhythmic effects of magnesium were due to (1) suppression of the functions of the sinus and AV nodes, and (2) shortening of the duration of the ventricular vulnerable period during the cardiac cycle and prevention of ventricular arrhythmias related reentry mechanism.