Prophylactic effect of magnesium sulfate on reperfusion-airhythmias was studied using a left anterior descending coronary artery occlusion followed by reperfusion in the isolated rat hearts. In the first part of the present work, we observed a bell-shaped relationship between the incidence of reperfusion-induced ventricular fibrillation (VF) and the duration of preceding ischemia. In the second part, the concentration of magnesium sulfate in perfusate was increased to 3.6, 4.8 and 6.0mmol/L, respectively, 1 min before coronary ligation, VF fell in a dose-dependent manner from its control total incidence of 100% to 82%, 73% and 18% (P