Little information exists regarding the effect of heart rate on arrhythmias induced by coronary reperfusion. We therefore evaluated the effects of different heart rates on arrhythmias following canine coronary artery occlusion and reperfusion. Dogs were paced at either 350 msec cycle length (171 bpm; n = 30) or 480 msec cycle length (125 bpm; n = 30). They were then subjected to a 20-minute occlusion of the proximal left anterior descending coronary artery, followed by sudden reperfusion. Ligated vessel perfusion bed size (myocardial "at risk") was measured with monastral blue and red dyes. The incidence of both occlusion and reperfusion arrhythmias correlated with the myocardium at risk. Dogs paced at 171 bpm had more ventricular ectopic depolarizations (37/1000 beats vs 8/1000 beats, p less than 0.01) and a higher incidence of ventricular tachycardia during occlusion than those paced at 125 bpm (67% vs 33%, p less than 0.05). Dogs paced at the faster rate also had a higher incidence of ventricular tachycardia (83% vs 60%, p = 0.08) and ventricular fibrillation (70% vs 40%, p less than 0.05) after reperfusion. Thus, heart rate can have a substantial effect on occlusion and reperfusion arrhythmias and should be considered when making therapeutic interventions and risk assessments in this setting.