The athlete's heart refers to a heart which has gained physiological adaptation mechanisms due to long term training in various sport activities. Sinus bradycardia, sinus arrhythmia, first degree atrioventricular block and second degree mobitz type I AV blocks are most common electrocardiographic findings in athlete's heart. In our case we observed first degree atrioventricular block with pathological PR interval values second degree mobitz type I (Wenckebach) atrioventricular block and accelerated junctional rhythm at the day time in a 13-year-old athlete playing football, exercising 21 hours a week. The clinical course of our patient indicates that such intense exercise may not always be beneficial to cardiovascular system in children.