Atrial fibrillation (AF) is a frequent arrhythmia complicating cardiac surgery, and generally occurs within the first week after surgery. Although there are some etiological mechanisms to explain the postoperative AF, the exact mechanisms of AF are not well clarified. In the present study, we would like to show the effect of MgSO(4) infusion on P duration and P dispersion, and its relation with AF complication in patients undergoing coronary artery bypass grafting. The patients were randomly allocated into two groups. Group A consisted of 93 patients to whom 1.5 g/day MgSO(4) infusion in 100 mL 0.9 NaCl solution (25 mL/hr) were applied the day before surgery, just after operation, and once daily for 4 days following surgery. Group B consisted of 55 control patients to whom 100 mL 0.9 NaCl solution (25 mL/hr) were applied at the same time points. Magnesium level was measured before the treatment and daily for the postoperative four days. As a result, AF developed in 2% of cases in group A, and in 36% of cases in group B (p < 0.001). Comparing the patients who developed AF, and who did not, no difference was detected with regard to baseline P max, P min, P dispersion and fourth day P min. But fourth day P max and P dispersion of patients who developed AF were significantly higher than who did not. Baseline Mg level were similar for those who developed AF, and who did not, but fourth day Mg level was significantly lower in AF group. As a result, our opinion that Mg infusion significantly decreased the incidence AF after cardiac bypass surgery was confirmed. And it's also clear that beneficial effect of MgSO(4) is associated with its decreasing effect on P dispersion.