Background and PurposeThe beat‐by‐beat fluctuation (dynamics) of heart rate (HR) depends on centrally mediated control of the autonomic nervous system (ANS) reflecting the physiological state of an organism. 5‐HT1A receptors are implicated in affective disorders,associated with ANS dysregulation which increases cardiac risk but their role in autonomic HR regulation under physiological conditions is insufficiently characterized.Experimental ApproachThe effects of subcutaneously administered 5‐HT1A receptor ligands on HR dynamics were investigated in C57BL/6 mice during stress‐free conditions and emotional challenge (recall of fear conditioned to an auditory stimulus and novelty exposure) using time domain and non‐linear HR analyses.Key ResultsPre‐training treatment with of 8‐OH‐DPAT (0.5 mg·kg−1, s.c.) prevented conditioned tachycardia in the retention test indicating impaired fear memory. Pretest 5‐HT1A receptor activation by 8‐OH‐DPAT (0.5 but not 0.1 and 0.02 mg·kg−1) caused bradycardia and increased HR variability. 8‐OH‐DPAT (0.5 mg·kg−1) lowered the unconditioned and conditioned tachycardia from ∼750 to ∼550 bpm, without changing the conditioned HR response to the sound. 8‐OH‐DPAT induced profound QT prolongation and bradyarrhythmic episodes. Non‐linear analysis indicated a pathological state of HR dynamics after 8‐OH‐DPAT (0.5 mg·kg−1) with ANS hyperactivation impairing HR adaptability. The 5‐HT1A receptor antagonist WAY‐100635 (0.03 mg·kg−1) blocked these effects of 8‐OH‐DPAT.Conclusions and ImplicationsPre‐training 5‐HT1A receptor activation by 8‐OH‐DPAT (0.5 mg·kg−1) impaired memory of conditioned auditory fear based on an attenuated HR increase, whereas pretest administration did not prevent the fear‐conditioned HR increase but induced pathological HR dynamics through central ANS dysregulation with cardiac effects similar to acute SSRI overdose.