Seven 24-hour ECG recordings and blood samples were taken within 3 weeks in 42 patients who had suffered an acute myocardial infarction (AMI). Ca, K and Mg concentrations in serum, and K and Mg in the erythrocytes, were determined by atomic absorption spectroscopy. One half of the patients were infused with 81 mval/day MgSO4 for 3 days. In patients who exhibited intense electrolyte alterations 10-20 days after AMI, there was a significantly higher rate in the frequency of couplets and/or tachycardia in the 2- to 20-day period after AMI. In patients infused with MgSO4, the fluctuation in serum electrolytes and the rate of arrhythmias were significantly reduced.