Recent studies of the phasic evoked cardiac response in the OR context have identified the deceleratory heart rate component with primary bradycardia. Cardiac deceleration thus appears to reflect stimulus registration, a preliminary process in OR elicitation. Situational factors may lead to an additional acceleratory component, probably reflecting cognitive processing demand. The present study investigated trials effects in these two components of a complex cardiac response, reflecting both stimulus and situational factors. The deceleratory component failed to show a simple trials effect, a finding compatible with the concept of stimulus registration. In contrast, the acceleratory component increased over trials, compatible with its hypothesized reflection of cognitive processing. © 1985, Psychonomic Society, Inc.. All rights reserved.