['To evaluate magnesium as a sole or adjuvant agent with currently used prophylactic drugs in suppressing postoperative atrial tachyarrhythmias (POAT) after coronary artery bypass graft (CABG) surgery.', 'Single-center prospective, randomized clinical trial.', 'University hospital.', 'Patients (n = 400) undergoing CABG surgery.', 'Patients were randomized among 6 prophylaxis regimens: (1) control (no antiarrhythmics), (2) magnesium only, (3) digoxin only, (4) magnesium and digoxin, (5) propranolol only, and (6) magnesium and propranolol. Patients randomized to a regimen including magnesium received 12 g given during 96 hours postoperatively. Patients in a digoxin regimen received 1 mg after cardiopulmonary bypass and 0.25 mg daily. Patients in a propranolol regimen received 1 mg intravenously every 6 hours until able to take 10 mg orally 4 times a day. Prophylaxis regimens were discontinued after 4 days postoperatively.', 'The primary outcome was a sustained POAT or discharge from the hospital. Control patients had an incidence of POAT (38%) not significantly different from patients in magnesium-only (38%), digoxin-only (31%), and magnesium with digoxin (37%) regimens. Patients treated with propranolol had a significant reduction in POAT. Nearly identical POAT rates in the propranolol-only (18%) and propranolol with magnesium (19%) groups support the lack of efficacy of magnesium in this trial. Study design allowed analysis of and showed a beta-blocker withdrawal effect in addition to suppressive benefit of postoperative beta-blockers.'] Is a beta-blocker , not magnesium , effective prophylaxis for atrial tachyarrhythmias after coronary artery bypass graft surgery?