Aims: Diagnostic work-up of athletes often reveals nonischaemic myocardial fibrosis, which is associated with cardiac dysfunction and malignant arrhythmias. Its isolated finding on magnetic resonance imaging is commonly attributed to preceding viral myocarditis. Previous murine studies have shown a negative effect of exercise early in the course of viral myocarditis. In this study, we investigate, for the first time, the impact of endurance exercise on myocardial fibrosis development and arrhythmogenicity in a murine viral myocarditis model. Methods & Results: Eleven-week-old male C57BL/6J mice (n=72) were randomised to 8 weeks of treadmill running (EEX) or without exercise (SED). After 2 weeks, animals received a single injection with either coxsackievirus B3 to induce acute viral myocarditis (CVB) or vehicle as control (PBS). Exercise resulted in a milder clinical disease course, with less weight loss and better preserved running capacity. In addition, mortality was lower in exercising myocarditis mice (11 vs. 27%), however without statistical significance (P=0.23). Exercise during myocarditis increased the occurrence of interstitial fibrosis (limited or extensive distribution) at sacrifice ( i.e. 6 weeks after inoculation)(82.4% in CVB-EEX vs. 56.3% in CVB-SED; P=0.049). Additionally, exercise enhanced development of perivascular and/or interstitial fibrosis with extensive distribution (64.7% & 64.7% in CVB-EEX vs. 50% & 31.3% in CVB-SED; P=0.048). CVB-EEX animals demonstrated more myocardial scars on average, albeit not significantly (1.9 vs. 1.2; P=0.19), with similar scar distribution. Despite manifest fibrotic remodelling, collagen and fibrogenetic genes were not upregulated in the myocarditis groups at the time of sacrifice. In vivo right ventricular programmed electrical stimulation showed comparable ventricular arrhythmia inducibility in the myocarditis groups (P>0.20). The longest arrhythmias and highest cumulative arrhythmia burden occurred in CVB-EEX, however, without reaching statistical significance when compared to CVB-SED (P=0.49). Conclusion: Endurance exercise during viral myocarditis enhances perivascular and interstitial fibrosis development, which may promote ventricular arrhythmogenicity.