Previous work has suggested that HR deceleration, commonly obtained in OR studies, reflected a preliminary process of stimulus registration rather than the OR itself. Research by the Laceys indicated that such a process was associated with a primary bradycardia, but they described this only over the first two poststimulus beats and not in an OR context. A preliminary study from this laboratory began the exploration of connections between these two phenomena by establishing their coexistence in an OR experiment. The present study aimed to examine the relationship between such HR deceleration and primary bradycardia in a complex cardiac response reflecting both stimulus and situational factors. The results suggest that the initial response of cardiac deceleration in the OR context may be nothing more than a primary bradycardia reflecting stimulus registration. © 1984, Psychonomic Society, Inc.. All rights reserved.