Irregular heartbeats and different forms of ventricular ectopic activity are a common occurrence among elite athletes with high contractile cardiac capacity. At the same time, experiments demonstrated that the electrical stimulation threshold, causing ventricular fibrillation, increases during adaptation to physical exe rcise, without the increase in the contractile cardiac capacity. The research purpose is to examine the dependence of ventricular fibrillation threshold and contractile cardiac capacity on intensity and duration of swimming sessions, as well as duration of the training period. Female Wistar rats were assigned to five groups: sedentary (S), training 1 (T1, low intensity), training 2 (T2, moderate intensity), training 3 (T3, long-term), training 4 (T4, exhaustive). At the end of the experimental period, the rats were anesthetized and their ventricles irritated with rectangular pulses of 10 ms duration, to determine the minimum current causing ventricular fibrillation. The cardiac capacity was assessed by the maximum pressure in the left ventricle, at full aortic-cross clamping. The ventricular fibrillation threshold was increased by 60% in T1, 57.5% in T2 and 74% in T3, but no difference in T4 was observed, compared with S. The pick pressure in the left ventricle after aortic cross-clamping in T1 and T2 was not enhanced, compared with S; in T3 and T4, however, it was significantly increased. Physical exercise training changed the ventricular fibrillation threshold and cardiac contractile capacity, independently of the intensity of exercise. The rise of the ventricular fibrillation threshold and its contractile capacity can be demonstrated during a long adaptation to moderate-term sessions of aerobic exercises.