Recent work in both animal and human studies emphasizes the value of magnesium in the maintenance of the functional and structural integrity of cardiac muscle. Both intracellular and extracellular magnesium concentrations can vary independently and the serum and red cell magnesium levels may not give an accurate account of intracellular cardiac magnesium deficiency. However, electrocardiographic studies of magnesium levels could provide an accurate index of intracellular cardiac magnesium levels. Twenty-four patients scheduled electively for mitral valve replacement were studied to evaluate the effect of slow releasing oral magnesium chloride on the QTc interval of the electrocardiogram. Although pretreatment QTc values in all patients were not significantly different, there was a highly significant difference between the control group and the treatment group after four days of preoperative treatment with oral magnesium chloride. During the postoperative phase of the trial, all patients developed a similar pattern of increase in QTc interval, reaching a peak at the end of the second day and followed by a decrease over the final two days. All patients who developed arrhythmias postoperatively had not been pretreated (primed) with oral magnesium chloride and had abnormal QTc intervals both before and after operation. The results of this study demonstrate the usefulness of oral magnesium chloride in reducing the QTc interval of the electrocardiogram and so protecting the myocardium against possible arrhythmias.