Recent studies of phasic response fractionation in the OR context have led to a theoretical development of the multiregister model of Barry (1979), which emphasizes the role of preliminary processes in OR elicitation. In this new model (e.g., Barry, 1981b), the novelty and magnitude of the stimulus are coded in separate registers which are similar in their operating characteristics to the hypothetical processes H and S of dual-process theory +(Groves & Thompson, 1970), and which interact to produce the OR. The present study examined the parallel between S and the stimulus magnitude register by manipulating anticipatory cardiac arousal between stimuli. A warned target stimulus was predicted to provide a context of enhanced cardiac arousal in comparison with a warned nontarget. The predicted S effect was obtained in the peripheral pulse amplitude response (PPAR), an index of the magnitude register. This and other results of response fractionation support the new model of preliminary OR processes. The results also bear upon recent findings of inconsistent habituation with PPAR measures. © 1982, Psychonomic Society, Inc.. All rights reserved.