BACKGROUND There are few reports on the precise electrocardiographic characteristics of acute myocarditis. The present study was focused on QRS voltage changes at the superacute stage of murine myocarditis. METHODS Serial electrocardiograms were recorded during the acute stage of viral myocarditis in mice, and then the cardiac pathology was examined. After recording baseline electrocardiograms, mice (n=235) were inoculated intraperitoneally with the encephalomyocarditis virus, resulting in severe myocarditis. Electrocardiograms were serially recorded until nine days after virus inoculation (superacute stage, days 3 to 6; acute stage, days 7 to 9). Changes in heart rate and QRS voltages were analyzed. RESULTS Serial electrocardiograms revealed that heart rates began to increase after day 3, and that the sum of the QRS voltages increased on day 3 and then decreased on days 7 to 9. Trivial mononuclear cell infiltrations and interstitial edema were most frequently found in mice at the superacute stage. CONCLUSIONS Transient increase of the QRS voltages at the superacute stage of myocarditis was demonstrated, which may be due to an increase in ventricular mass caused by interstitial edema at this stage.