In patients with acute myocarditis, arrhythmias constitute one of the crucial risk factors for morbidity and mortality. To clarify the incidences and the chronological features of arrhythmias and their correlation with the histopathology of the heart, we conducted continuous electrocardiography in an animal model of viral myocarditis, and light microscopy of the heart with special reference to the conduction system. Forty weanling C3H/He mice were divided into an inoculated group (IG) and a control group (CG), of 20 mice each. IG was injected intraperitoneally with coxsackie B3 virus and CG was injected with virus-free culture medium. Thin electrodes were implanted in their chest walls and connected with an electric impulse transmitting device so that they could move freely. ECGs were recorded continuously without anesthesia up to the 14th postinoculation day. The atrial and ventricular myocardium and the conduction system were studied by light microscopy. In CG, no arrhythmia was noted. In IG, various kinds of arrhythmias were documented. The incidences of the arrhythmia in the 20 mice were: sinus arrest 80%, second or third degree atrioventricular (AV) block 30%, premature atrial complexes 30%, premature ventricular complexes 20%, and ventricular tachycardia 10%. Arrhythmias were usually transient and recurrent, and the majority of them developed between the 6th and 13th day, when the histologic changes of the heart were greatest. There seemed to be a correlation between the kind of arrhythmias and the myocarditic lesions. Mice with sinus arrest or AV block developed histopathologic changes in the sinus node or AV conducting tissue, respectively. In the early stage of myocarditis, no inflammatory changes were apparent in the sinus node in the mice with sinus arrest.