Serum magnesium concentrations and the rate of urine magnesium excretion were studied in 24 patients with suspected acute myocardial infarction (AMI). Blood and urine samples were taken on admission, at three-hour intervals for the first 24 hours after admission, and every eight hours for the next 24 hours. Thirteen of the patients were found to have AMI, and the 11 who did not have AMI served as a control. During the first 32 hours, the AMI group had significantly low serum magnesium concentrations. The serum magnesium concentrations were unchanged in the control group. Results of the urine samples disproved our hypothesis that the drop in serum magnesium concentrations was due to an increased renal magnesium loss. These results indicate a magnesium migration associated with AMI, from extracellular to intracellular space.