A magnesium (Mg++) retention test was performed in 19 patients with acute myocardial infarction (AMI) 4-11 days after admission to the coronary care unit. The retention of Mg++ was 45 +/- 23% of the 30 mmol given intravenously. It has been proposed that a retention of more than 20% represents Mg++ deficiency. The mononuclear cell Mg++ concentration before the retention test was on an average slightly higher in the AMI patients than in 25 healthy volunteers (72.5 +/- 24.2 vs. 62.9 +/- 9.2 mumol g-1 protein) indicating no Mg++ depletion in the first group. The reason why patients during the phase of AMI show an increased retention of Mg++ is unknown, but changes in concentrations of several hormones and a reduction in blood glucose could be of importance. Serum concentrations of Mg++ were lower on admission than after 4-11 days. These initial reductions are probably due to increased concentrations of circulating catecholamines during the early hours of AMI.