It has been well established that in acute myocardial infarction (MI) many patients display low serum magnesium (Mg). This is associated with complex ventricular arrhythmias. The question arises whether predischarge arrhythmias occurring late after MI might also be related to Mg imbalance. In 118 patients subjected to heart rhythm 24 h Hotter monitoring in the second or third week after MI, we investigated (1) the relationship between serum Mg, urinary Mg loss, and ventricular arrhythmias, and (2) the effect of Mg supplementation on heart rhythm disturbances. In patients with undisturbed rhythm or monomorphic ventricular ectopic beats (VEB) (Lown 0-2; n = 84), mean serum Mg level (mg% ± SD) was 1.83 ± 0.21, whereas in patients with multifocal VEB, pairs, or nonsustained ventricular tachycardia (VT) (Lown 3–4; n=34) serum Mg was decreased to 1.68 ± 0.27 (p < 0.01). Serum Mg normal range in our laboratory is 1.7–2.6 mg%. The lowest serum Mg reaching 1.55 ± 0.27 was found in nonsustained VT (Lown 4 b) subgroup (n = 14). Urinary Mg loss measured in 81 patients was more pronounced in those with Lown 3–4 arrhythmias (n=26) than with Lown 0–2 (n = 55). The daily values were 73 ± 22 and 54.4 ± 26 mg, respectively (p < 0.001). Thirteen patients with complex arrhythmias and low serum Mg received Mg supplementation (MgSO4, 8 g in 500 ml 5% glucose intravenously during 24 h). This resulted in restoration of almost undisturbed rhythm in 10 subjects. In conclusion, a number of patients in late phase of MI displayed hypomagnesemia concomitant with augmented Mg excretion. These alterations were related to the occurrence of arrhythmias: die most threatening heart rhythm disturbances were accompanied by the lowest serum Mg and the highest Mg excretion. Our preliminary observations suggest that supplementation with MgSO4 may be a reasonable approach to the treatment of predischarge complex ventricular arrhythmias after MI. This should be verified in a controlled trial.