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The time course of ventricular arrhythmia following reperfusion was investigated in 12 dogs undergoing a 2-hour coronary artery occlusion followed by reperfusion. At days 1, 3 and 7 following coronary reperfusion, the cardiac rhythm was monitored with a 24-hour Holter electrocardiographic recorder. Six dogs with sham operations were also studied with Holter monitoring at the same time periods as a control group. This showed that, following coronary reperfusion, all 12 dogs (group A) developed frequent premature ventricular complexes (PVCs) at day 1, with a total count (beats per hour, bph) of 1,806 +/- 390 (mean +/- SD). The PVC count decreased to 298 +/- 96 at day 3 (p less than 0.001) and 0.4 +/- 0.1 at day 7 (p less than 0.001). The incidence of spontaneous sustained ventricular tachycardia (VT) at day 1 was 100%; 1 of them deteriorated to ventricular fibrillation (VF). At day 3, 9 of the remaining 11 dogs (82%) had recurrent VTs (p greater than 0.05 vs day 1) which were of shorter duration and a slower rate. One of the 11 dogs died at day 5 and the remaining 10 dogs survived to day 7, with all the VTs subsiding (p less than 0.001 vs day 1 or 3). In the control group, only isolated PVCs were observed at day 1 (7 +/- 3 bph, p less than 0.001 vs group A) and no spontaneous VT or VF was noted. We conclude that spontaneous ventricular arrhythmias were observed in all dogs with coronary reperfusion following a 2-hour coronary artery occlusion and the arrhythmias could subside in 1 week of those survived. Only rarely did VT deteriorate to VF.