Susceptibility of rat myocardium to reperfusion-induced arrhythmias after sustained (4 h) and brief (10 min) durations of regional ischaemia in relation to structurally-related impairment of heart function was studied in isolated Langendorff-perfused rat hearts. Reperfusion arrhythmias after 4 h ischaemia were characterized by low severity accompanied by only partial restoration of coronary flow upon reperfusion (no-reflow). Electron microscopy examination of the ischaemic hearts revealed severely ultrastructure of cardiac myocytes and capillary endothelium. These deteriorations were not reversed upon reperfusion. On the contrary, brief ischaemia resulted in high susceptibility of the heart to severe arrhythmias upon reperfusion with persisting hyperaemia. Ischaemia-induced minor changes in myocardial ultrastructure were reversed upon reperfusion. In conclusion, sustained ischaemia in the rat heart with absent collaterals renders myocardial tissue non-viable with consequent impaired recovery of heart function and loss of excitability of the myocardium upon reperfusion. Accordingly, high susceptibility to arrhythmias may indicate the preservation of viable myocardial cells.