Personality characteristics, e.g. aggressiveness, have long been associated with an increased risk of cardiac disease. However, the underlying mechanisms remain unclear. In this study we used a rodent model for characterizing cardiac autonomic modulation in rats that differ widely in their level of aggressive behavior. To reach this goal, high-aggressive (HA, n = 10) and non-aggressive (NA, n = 10) rats were selected from a population (n = 121) of adult male Wild-type Groningen rats on the basis of their latency time to attack (ALT, s) a male intruder in a resident-intruder test lasting 600 s. In order to obtain information on their cardiac autonomic modulation, ECG recordings were subsequently obtained via radiotelemetry at rest, during stressful stimuli and under autonomic pharmacological manipulations, and analyzed by means of time- and frequency-domain indexes of heart rate variability. During resting conditions, HA rats (ALT<90 s) displayed reduced heart rate variability, mostly in terms of lower vagal modulation compared to NA rats (ALT>600 s). Exposure to stressful stimuli (i.e. restraint and psychosocial stress) provoked similar tachycardic responses between the two groups. However, under stress conditions HA rats displayed a reduced vagal antagonism and an increased incidence of tachyarrhythmias compared to NA rats. In addition, beta-adrenergic pharmacological stimulation induced a much larger incidence of ventricular tachyarrhythmias in HA rats compared to NA counterparts. These findings are consistent with the view that high levels of aggressive behavior in rats are associated to signs of cardiac autonomic impairment and increased arrhythmogenic susceptibility that may predict vulnerability to cardiac morbidity and mortality.