There have been many reports about ventricular arrhythmias during acute coronary occlusion. Nevertheless, it is only recently that interest has been taken in the occurrence of ventricular arrhythmia after reperfusion following coronary occlusion. To investigate the mechanism of the latter kind of arrhythmia, we studied the effect of changing the duration of occlusion time on the recovery time courses of the VMRT (Ventricular Multiple Response Threshold), and of the A-V differences in the serum K+ concentration across the heart. The time course of delta K+ recovered soon after reperfusion, while changes in VMRT needed more time for recovery to the initial state. Concerning heart rate, blood pH, and the levels of Na+, Cl-, and Ca++, no significant changes were detected. There was no relation between the time courses of VMRT and those of the A-V differences in serum K+. Consequently, time courses in VMRT were dependent upon the duration of coronary occlusion time. A possible explanation for these results may be that the longer the duration of the preceding occlusion time, the more severe the myocardial damage due to myocardial ischemia.