<p>Laboratory research suggests that the attenuation of defensive responses during threat exposure relies on the inhibition of subcortical defense circuits orchestrated by the prefrontal cortex. This process may be reflected by increased vagal efferent activity on the autonomic nervous system. Here, we tested whether cardiac vagal efferent activity, indicated by vagally-mediated heart rate variability (vmHRV), is associated with reduced defensive responses during in-vivo threat exposure and whether it may serve as a physiological index of multi-level exposure responding. Thirty-three women with excessive fear of spiders underwent standardized exposure, where they first approached a living tarantula (approach phase) and subsequently progressed through stages of increasingly direct interactions with the spider (threat imminence task). Indices of cardiac vagal activity (vmHRV) and overall cardiac autonomic arousal (heart rate) were measured alongside self-reported fear and avoidance behavior (distance to spider, performance in threat imminence task). During approach, self-reported fear increased until it reached a stable ceiling. Concurrent vmHRV leveled accordingly, with greater activity being related to lower fear and reduced avoidance during the subsequent threat imminence task. From the very first minute, cardiac vagal activity was associated with the participants’ ability to physically engage with the fear cue during threat imminence task, irrespective of covariations in self-reported fear. Our results suggest that cardiac vagal activity may represent inhibition of fear and avoidance during exposure to perceived threat. Cardiac vagal activity, indexed by vmHRV, could therefore represent an index of within-session multi-level responding to exposure and a promising target for adjuncts to promote fear reduction.</p>