BACKGROUND Moderate exercise training has been shown to decrease sudden cardiac death post myocardial infarction. However, the effects of intensive exercise are still controversial. METHODS AND RESULTS Fourteen myocardial-infarcted rats were divided into sedentary (n=8) and intensive training groups (n=6) and 18 sham control rats to sedentary (n=10) and intensive training groups (n=8). Heart rate variability was obtained at weeks 1 and 8. The inducibility of ventricular tachycardia/fibrillation was assessed in a Langendorff system. Fast Fourier transforms were applied on the recorded ventricular tachycardia/fibrillations. Training reduces low to high frequency ratio of heart rate variability at week 8 compared with that at week 1 (P<0.05). In isolated hearts, the probability for ventricular tachycardia/fibrillation was decreased from 4.5±0.8% in sedentary controls to 1.56±0.2% in intensive training controls (P<0.05) and from 13.5±2.1% in the sedentary group to 5.4±1.2% in the intensive training group (P<0.01). Moreover, the pacing current required for ventricular fibrillation induction in the trained groups was increased following exercise (P<0.05). Fast Fourier transform analysis of ECG findings revealed an exercise-induced ventricular fibrillation transition from a narrow, single-peak spectrum at 17 Hz in sedentary controls to a broader range of peaks ranging from 13 to 22 Hz in the intensive training controls. CONCLUSIONS Intensive exercise in infarcted rats leads to reduced ventricular fibrillation propensity and is associated with normalization of refractoriness and intrinsic spatiotemporal electrical variations.