Study objective — The aim was to investigate the effect of preconditioning on reperfusion induced arrhythmias in the isolated working rat heart. Design — Rat hearts were perfused with modified Krebs-Henseleit bicarbonate buffer. A 15 min period of global ischaemia was performed followed by a reperfusion period of 5 min. This procedure was repeated three times. Hearts were divided into the control group (n = 10) and the free radical scavenger group (n = 11). Hearts in the latter were reperfused in the presence of scavengers (SOD 3.5 × 104U·litre−1 and catalase 5.0 × 105 U·litre−1). Experimental material — Hearts were obtained from male Sprague-Dawley rats (270-350 g). Measurements and main results — The incidence of reperfusion induced ventricular fibrillation in the control group decreased from 100% to 70% (NS) and to 40% (pConclusions — (1) The protective effect of preconditioning on reperfusion induced arrhythmia is evident in the isolated rat heart. (2) Addition of free radical scavengers reduced the incidence of reperfusion arrhythmia. However, the protective effect of preconditioning was lost.