In order to observe the change of epicardial ECG(Eep), left ventricular pressure, left ventricular dp/dt and the development of arrhythmia during regional myocardial ischemia and reperfusion, proximal LAD was ligated for 30 minutes and reperfused suddenly for 30 minytes in eleven mongrel dogs which were grouped into control(n=6) and diltiazem(n=5) group. In diltiazem group, diltiazem infusion was started 10 minutes prior to reperfusion with the speed of 0.02mg/kg/min for 25 minutes. The amount of injury current was measured from TQ segment and ST segment changes of Eep, and its effect on the incidence of reperfusion arrhythmia was evaluated. Eep, LV pressure, LV dp/dt and ECG were simultaneously recoreded with the paper speed of 100mm/sec at predetermined time intervals, and 6 channel ECG(standard lead I, II, III, AVR, AVL, AVF) was recorded continuously with paper speed of 10mm/sec throughout the experiment. The results were as follows ; 1) After ligation of LAD, the polarity QRS of Eep changed to show monophasic shape from 3-4 minutes, TQ segment depressed to reach minumum level at 4-7 minutes and ST segment elevated to reach maximum level at 4-5 minutes. These changes recovered rapidly to pre-ligation state after reperfusion, and this tendency was not affected by diltiazem. 2) The absolute value of LV dp/dt max and LV dp/dt min decreased 10% at 2-4 minutes after LAD ligation, and began to recover from 7 minutes after reperfusion to reach peak recovery value at 20 minutes after reperfusion in control group. In diltiazem group, it decreased 15% after diltiazem infusion and began to recover from 1 minutes after reperfusion to reach peak recovery value at 7 minutes after reperefusion. 3) Ischemic ventricular fibrillation was observed at the time of maximum TQ depression and ST segment elevation and 4 out of 6 events were developed within 5 minutes after LAD ligation. The cases with Isch-Vf developed Rep-Vf without exception, which was observed in 8 out of 11 cases and was noted within 1 minutes after reperfusion except one. 4) Maximum ST elevation was significantly higher in group with Rep-Vf then in group without Rep-Vf(Rep-Vf(+);18.5+/-11.1, Rep-Vf(-);10.3e+/-6.9, p<0.05), and also maximum ST elevation was significantly higher in group with both Isch-Vf and Rep-Vf then in group with only Rep-Vf(Isch-Vf+Rep-Vf;28.5+/-7.8, Rep-Vf only;10,5+/-4.7, P<0.01). 5) The incidende of reperfusion ventricular fibrillation was 83% in control group(5 out of 6) and 60% in diltiazem group(3 out of 5), but the inhibitory effect of diltiazem on the reperfusion Vf could not be confirmed due to the difference of the incidence of ischemic Vf between the two groups(control group;67%(4 out of 6), ditiazem group;20%(1out of 5)). In conclusion, maximum injury current developed 4-7 minutes after coronary artery ligation, and maximum ST elevation value was significantly related with the development of ischemic Vf and reperfusion Vf, and the inhibitory effect of diltiazem on the reperfusion ventricular fibrillation could not be confirmed in this study.