Background: Magnesium is one of the important intracellular cations. Hypomagnesemia is common after an open heart surgeryand may affect the development of posoperative arrhythmias. The aims of this study were to identify 1) the severity of the hypomagnesemia 2) the adequate dose of the magnesium replacement and 3) the effect of magnesium replacement on the postoperative arrhythmias. Material and Method: The serum magnesium level was measured in 20 patients in whom magnesium was replaced postopertively(6gm at the operative day 4gm at the 1st postoperative day and 2gm at the 2nd postoperative day) and compared with that of the 13 patients in whom magnesium was not replaced postopertively The serum magnesium level was normalized after magnesium replacement. We analyzed the development of arrhythmias in the patients groups who did not receive magnesium and were operated on between Oct. 1994 and Oct. 1995(Group I; n=206) and who received the magnesium postoperatively and were operated on between Nov. 1995 and Aug. 1996(Group II; n=133) Result: There were no differences in the preoperative risk factors and the rate of postoperative supraventricular or ventricular tachyarrhythmia occur-rences irrespectivel of the magnesium replacement. Magnesium replacement could prevent the aggrevation of the occurrence of postoperative arrhythmias in high risk groups of ventricular tachyarrhytnmia in old age but magnesium could not prevent postoperative arrhythmia in other high risk groups. Conclusion: The magnesium replacement after open heart surgery could prevent the development of ventricular tachyarrhythmias especially in old age groups but could not prevent atrial tachyarrhthmias.