Convincing proof is now available that reperfusion of the ischaemic myocardium results in release of oxygen-derived free radicals (ODFR). Experimental data support the hypothesis that ODFR play a pathological physiological role in the development of post-ischaemic systolic dysfunction (stunning), microcirculatory disturbances and reperfusion arrhythmias. In addition, the free radicals appear to modify the energy metabolic conditions after reperfusion. The results from investigations of the effects of ODFR on the extent of myocardial infarction are difficult to interpret as, in various experimental series in which formation of ODFR is reduced to a minimum, the infarct reducing effect has not been found to be in agreement with similar durations of ischaemia. The varying results concerning the extent of the infarcts suggests that other factors e.g. collateral circulation and balance between oxygen supply and oxygen requirements play a dominating role in these problems. It has not yet been elucidated whether ODFR are of significance in man in connection with reperfusion of the ischaemic myocardium.