BACKGROUND: Ischemic preconditioning(a prior short period of coronary artery occlusion) has been known to have protective effects on ischemia-induced myocardial injury. The purpose of this study was to investigate the effects of hypoxic preconditioning or ischemic preconditioning on the reperfusion-induced ventricular arrhythmias after a prolonged period of coronary artery occlusion in the cat hearts. METHOD: Thirty-six cats were anesthetized with a-chloralose(60mg/kg, I.P.) and prepared for left anterior descending coronary artery(LAD) occlusion after thoracotomy. A 10 minute occlusion of LAD was used with a subsequent 3 minute reperfusion period as ischemic-preconditioning before a 20 minute occlusion of LAD and subsequent reperfusion in 10 cats. Hypoxic-preconditioning was done according to the identical procedure as ischemic-preconditioning but using hypoventilation and normal ventilation instead of occlusion and reperfusion, respectively, in 12cats. The remaining 14 were used as controls following occlusion of LAD for 20 minutes and subsequent reperfusion. RESULTS: Two preconditionings significantly reduced the severity of ventricular arrhythmias during the reperfusion period following a 20 minutes occlusion of LAD compared with controls. Ischemic-proeconditioning reduced the frequency of ventricular premature beats(VPBs) during a 20 minute occlusion of LAD with absence of significant fatal ventricular arrhythmias such as ventricular thachycardia or fibrillation compared with controls. Hypoxic-preconditioning failed to reduced the frequency of VPBs during a 20 minute occlusion of LAD where as ventricular fibrillation was totally suppressed and ventricular tachycardia occurred in one cat. CONCLUSION: These data suggest that 1) not only ischemic-preconditioning but also hypoxic-preconditioning alleviates the reperfusion-induced arrhythmias : 2) the mecnanism of the reperfusion-induced arrhythmias is different from that of the ischemia-induced arrhythmias : 3) hypoxic-preconditioning allows one to perform further studies regarding the mechanisms of preconditioning.