One hundred patients undergoing elective cardiac operations were randomized into placebo (n = 54) and magnesium (n = 46) groups. The magnesium group received six doses of 2.4 g (19.2 mEq) magnesium sulfate intravenously in the first 24 hours after the cardiac operation. The magnesium group had higher serum magnesium concentrations postoperatively (1.09 +/- 0.20 versus 0.75 +/- 0.13 mmol/L; p < 0.0001), postoperative day 1 (1.49 +/- 0.34 versus 0.70 +/- 0.12 mmol/L; p < 0.0001) and postoperative day 2 (0.96 +/- 0.19 versus 0.76 +/- 0.07 mmol/L; p < 0.0001). Patients in the magnesium group had a lower incidence of ventricular tachyarrythmias (VTs) (17.3% versus 51.9%; p = 0.0006), less need for treatment (6.5% versus 20.3%; p < 0.0001), fewer VT episodes/patient (0.3 +/- 0.8 versus 1.39 +/- 1.9; p < 0.0001), and a reduction in the severity of VTs as measured by the modified Lown grade (p = 0.0002). No differences were demonstrated with respect to supraventricular tachyarrythmias. The magnesium group had reduced absolute creatine kinase-MB levels (5.3 +/- 4.2 versus 28.4 +/- 28 IU/L; p = 0.001) as well as creatine kinase-MB fraction (0.01 +/- 0.02 versus 0.05 +/- 0.04; p = 0.001) on postoperative day 1. Serum magnesium concentrations were lower during VTs than during periods of sinus rhythm (0.75 +/- 0.75 versus 1.02 +/- 0.35 mmol/L; p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)