AbstractMaternal aggression of lactating rats was used to induce acute social stress in a resident‐intruder paradigm. Behavioral and electrocardiographic responses of male intruders were simultaneously recorded. Cardiac electrical activity was measured by means of a telemetry system which ensured preservation of intruder's behavioral repertoire. Behavioral response to maternal attack was evaluated in terms of percent time spent in passive/submissive (p/s) and active/non‐submissive (a/ns) patterns. This allowed us to classify the intruders as submissive, non‐submissive, and intermediates. During the test, a significant decrease of average R‐R interval (R‐R) compared to baseline conditions and the occurrence of arrythmias of different nature were found. Ventricular arrhythmias, including premature ventricular beats and ventricular tachycardias, were present in all the intruders while R‐R fluctuations and II degree atrioventricular (A‐V) blocks only appeared in submissive and intermediate rats. Most ventricular arrhythmias occurred within the upper range of heart rate and were preceded by periods of low R‐R variability. These electrocardiographic events were temporally associated with a/ns behavioral patterns and were related to an increased prevalence of sympathetic activity. On the other hand, R‐R fluctuations and II degree A‐V blocks preceded by high R‐R variability were synchronized with p/s behavior and were ascribed to a predominant inhibitory effect of vagal activation which becomes more pronounced as the level of sympathetic activity increases.