BACKGROUND Atrial and ventricular arrhythmias are among the most common complications after coronary artery bypass graft (CABG) surgery. It is known that cardiopulmonary bypass reduces serum magnesium level. In this study, we evaluated the relationship between total blood magnesium level (TMG) and the incidence of perioperative arrhythmias. METHODS TMG was measured in patients who were scheduled for CABG on three occasions: just before anesthesia, on intensive care unit (ICU) arrival and on the first morning after operation. Patients were evaluated for primary cardiac rhythm, serum creatinine, urine output in operating room and diuretic therapy. Supplemental magnesium (SMG) was also recorded in operating room and ICU. Patients were then evaluated for the rate and kind of arrhythmia occurring during the next 3 days. RESULTS Mean TMG level in 170 cases was 2.2 (0.5), 2.6 (0.6) and 2.4 (0.6) mg/dl on three occasions respectively. 53 patients developed post-operative arrhythmia (31%) [Atrial Fibrillation (AF) (7.1%), Non-AF Supraventricular arrhythmia (14.7%) and Ventricular arrhythmia (16.5%)]. Although there was a significant difference between TMG on three occasions (P <0.001), all values were within normal range. Although TMG was higher in arrhythmic patients compared to non- arrhythmics (2.26 vs. 2.14), both values were in normal range and there was no significant difference between two groups. CONCLUSIONS This study shows that routine magnesium administration has no significant effect on serum magnesium level. We conclude that though routine regimen of magnesium administration has no effect on incidence of perioperative arrhythmia, it is probably necessary for maintaining normal magnesium level.