The effect of magnesium on potassium fluorocarbon cardioplegia was studied in 20 rabbits. Isolated hearts with ascending aortas were perfused with oxygenated and cooled (+4 degrees C) fluorocarbon solutions. In 10 experiments, the solution contained 20 mmol/l of potassium, in 10 further experiments, 20 mmol/l of potassium and 15 mmol/l of magnesium. Ultrastructural preservation of the myocardium was studied using transmission electron microscopy. In addition, the cation contents of the myocardium were studied by flame atomic absorption spectrophotometry (calcium, magnesium) or electrothermal atomic absorption spectrophotometry (copper). After 120 minutes of ischaemia, areas of moderate or severe ultrastructural myocardial cell damage were observed in both groups but severe damage was more extensive and commoner in the group with no access to magnesium. Preservation of the capillary endothelium was also worse in that group. There was a highly significant increase in magnesium content, a decrease in calcium content and an increase in the magnesium/calcium ratio following magnesium-potassium fluorocarbon infusion. The copper content remained unchanged. A protective effect of magnesium was observed. The electrolyte content of cardioplegic solutions also alters the electrolyte content of the myocardium. This may be important in reperfusion.