Ischaemia of the myocardium leads to necrosis unless oxygen supply is restored but it has only recently been realised that reperfusion is not without danger. The greatest rate of myocardial damage, as measured by mitochondrial function, occurred during the first 5 minutes of reperfusion in rat hearts subjected to normothermic ischaemic cardiac arrest. Addition of desferrioxamine to the perfusate after 5 minutes of reperfusion did not reverse the mitochondrial damage. It is therefore concluded that desferrioxamine prevents mitochondrial damage caused by ischaemia-reperfusion but does not reverse the damage already present.