The present study investigated the serial changes in serum magnesium (Mg) and erythrocyte concentration of Mg in patients with acute myocardial infarction (AMI) and the relationship between these changes and left ventricular ejection fraction (LVEF) at 1 month after the onset of infarction. The study group comprised 26 patients with AMI (mean age, 57.9+/-8.9 years). Serum Mg and erythrocyte Mg were measured on hospital days 1, 2, 4, 7 and 21. The change in erythrocyte Mg during the acute phase was calculated as a ratio: [(erythrocyte Mg at day 2)-(erythrocyte Mg at day 1)]/(erythrocyte Mg at day 1). The change in serum Mg was calculated similarly. The following results were obtained. (1) Serum Mg tended to increase from the onset of myocardial infarction (day 1: 1.86+/-0.19, day 2: 1.93+/-0.22, day 4: 2.17+/-0.23; day 7: 2.25+/-0.20; day 21: 2.12+/-0.15 mg/dl). (2) Erythrocyte Mg on day 2 and day 4 showed a significant decrease compared with day 1 (day 1: 2.45+/-0.40, day 2: 2.09+/-0.41, day 4: 2.07+/-0.37, day 7: 2.22+/-0.33, day 7: 2.34+/-0.28 mg/dl per 400x10(4)/mm3 cells). (3) A significant positive correlation was observed between the change in serum Mg and LVEF (r=0.55, p<0.05), and a significant negative correlation was observed between the change in erythrocyte Mg and LVEF (r=-0.57, p<0.05). Thus, it was concluded that an extracellular shift in intracellular Mg occurred during the first 2 days after the onset of myocardial infarction. This responsive increase in the extracellular Mg level may be an important factor for maintaining left ventricular function in patients 1 month after the onset of AMI.