For the purposes of the European double blind and randomized study 27 patients with acute myocardial infarction underwent thrombolysis with rt-PA (60 mg over 90 minutes i.v.) or placebo. To evaluate whether arrhythmias, especially ventricular arrhythmias indicate coronary reperfusion after thrombolysis a 24 hour Holter monitoring was performed from the beginning of the rt-PA or placebo infusion. Typical reperfusion arrhythmias were thought to be idioventricular rhythms (rate less than 110/min), ventricular tachycardia (rate greater than 110/min) or bradycardic rhythm disturbances (rate less than 50/min). The effect of thrombolysis on reperfusion of the infarct related artery was evaluated 90 minutes after the infusion by coronary angiography. After 90 minutes of rt-PA or placebo infusion in 16/16 patients treated with rt-PA and 2/11 patients, who received placebo, was the infarct artery patent. 16/18 patients with a patent artery presented a total of 105 arrhythmic events. 47% of the arrhythmias obviously due to reperfusion were classified as idioventricular rhythms. In contrast only 3/9 patients with an occluded infarct artery presented 25 arrhythmic events. The time of occurrence was not different during the running rt-PA infusion compared to placebo. The following interval up to 24 hours showed no difference in incidence and type of the arrhythmias. No relationship was found between reperfusion arrhythmias and salvage of myocardium during 90 minutes of rt-PA or placebo infusion.