Depressed mood is an independent risk factor for cardiac morbidity and mortality, but the exact physiological mechanisms linking depression and cardiac dysfunction are not yet clarified. In this study, we examined cardiac electrophysiological properties in a rat model of depression based on 12 sessions of social defeat over a period of 4 weeks (SD, n=8); controls were exposed to an empty unfamiliar cage with clean bedding (n=8). Social defeat induced robust tachycardic response and vagal withdrawal, which did not habituate across repeated episodes. SDs displayed slowing down of body weight gain and reduction in the daily amplitude of heart rate, body temperature and physical activity. At the end of chronic stress, rats were submitted to restraint and forced swim test: although SDs showed larger immobility time in the forced swim test, cardiac response to restraint did not reveal group differences. At sacrifice, epicardial potential mapping was performed. While myocardial excitability and impulse conduction velocity were similar in the two groups, QT interval was markedly shortened (-27%) in SDs. Moreover, ERP was smaller in defeated rats (88?5 vs 102?4 ms), indicating reduced myocardial refractoriness, that could represent a long-lasting pro-arrhythmic effect of chronic stress. In conclusion, prolonged social challenge induced behavioral and physiological symptoms of depression, that were associated with cardiac electrophysiological signs of increased risk of arrhythmias.