Early period of reperfusion of ischemic myocardium is associated with a high incidence of severe tachyarrhythmias including ventricular tachycardia and fibrillation (VT and VF). Free oxygen radicals (FOR) have been identified as one of the principal factors responsible for reperfusion-induced events. However, their role in arrhythmogenesis is not clear. In the present study, in isolated Langendorff-perfused rat hearts subjected to 30 min global ischemia, the onset of reperfusion induced 100% incidence of both VT and VF with their gradual cessation over 5 min of reperfusion. Generation of H2O2 in the myocardium in the first minutes of reperfusion was visualized by means of cerium cytochemistry and confirmed by X-ray microanalysis. The mechanism of the arrhythmogenic effect of FOR may involve inhibition of the sarcolemmal Na+/K+-ATPase, as demonstrated in the rat heart sarcolemmal fraction subjected to FOR-generating system (H2O2 + FeSO4).