Desmoglein-2 mutations are detected in 5-10% of patients with arrhythmogenic right ventricular cardiomyopathy (ARVC). Endurance training accelerates the development of the ARVC phenotype, leading to earlier arrhythmic events. Homozygous mutant mice develop a severe ARVC-like phenotype. The phenotype of heterozygous mutant ( ) or haploinsufficient ( ) mice is still not well understood. To assess the effects of age and endurance swim training, we studied cardiac morphology and function in sedentary one-year-old and mice and in young mice exposed to endurance swim training. Cardiac structure was only occasionally affected in aged and mice manifesting as small fibrotic foci and displacement of Connexin 43. Endurance swim training increased the right ventricular (RV) diameter and decreased RV function in mice but not in wild types. hearts showed increased ventricular activation times and pacing-induced ventricular arrhythmia without obvious fibrosis or inflammation. Preload-reducing therapy during training prevented RV enlargement and alleviated the electrophysiological phenotype. Taken together, endurance swim training induced features of ARVC in young adult mice. Prolonged ventricular activation times in the hearts of trained mice are therefore a potential mechanism for increased arrhythmia risk. Preload-reducing therapy prevented training-induced ARVC phenotype pointing to beneficial treatment options in human patients.