A catheter through which perfusion was performed with oxygenated saline (2.1 ml/min) was introduced into the right coronary artery ostium of the rat right ventricle that had been isolated during cardioplegia. Superperfusion (12 ml/min) was simultaneously made. Termination of the perfusion caused antiarrhythmias at minutes 6 to 28 of ischemia. The highest likelihood of occurrence of such arrhythmias was observed on minutes 16-20 (premature beats being seen in 86% of the experiments, extrastimulus-induced tachycardias in 75%, spontaneous tachycardias in 25%). Reperfusion was made at 3, 5, 7, 10, 13, 15, 20, 30 and 60 min following ischemia (n = 7 in each case). The occurrence of reperfusion arrhythmias is likely to be related to the duration of ischemia with the highest likelihood of 20 minutes after ischemia (tachycardia and fibrillation were observed in 100 and 71%, respectively).