Reperfusion arrhythmias were studied in a group of 20 patients submitted to coronary thrombolysis in the early hours of acute myocardial infarction. Arrhythmias were observed in 15 (75%) patients and consisted of ventricular arrhythmias and/or sinus bradycardia; 11/13 patients with reperfusion ventricular arrhythmias had the same type of arrhythmias before the procedure. This study group was compared to another group of 22 patients with acute myocardial infarction treated conventionally. There was no difference between both groups in regard to the incidence and type of ventricular arrhythmias. Sinus bradycardia only occurred during reperfusion in the study group and was significantly predominant in this group when compared with control group.