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Following classical conditioning to a shock-reinforced tone CS (T,+), heart rate (HR) of three groups of rats was examined in response to a new reinforced tone (Tz+) and a nonreinforced light (L-) CS given during conditioned inhibition (Tz+ vs. TzL-), discrimination conditioning (Tz+ vs. L-), or explicitly unpaired (Tz/L-vs. US alone)procedures. Decelerative HR reactions occurred to the reinforced T 1+ and T z+ ess. To the respective nonreinforced ess, the conditioned inhibition group displayed diminished but sizable HR deceleration, the discrimination group showed near-zero responding, and the explicitly upaired group showed HR acceleration. Subsequent reversal conditioning to L was retarded in the conditioned inhibition and explicitly unpaired groups relative to the discrimination group. Group differences on combined-cue (T,/L-) trials were not found. Both the HR responses during inhibitory training and the reversal-conditioning impairments suggest that inhibition may have been established to L-in the conditioned inhibition and explicitly unpaired groups. Cunningham, Fitzgerald, and Francisco (1977) reported that directionally opposite heart rate (HR) responses were produced in restrained rats by varying the contingency relation between a CS and a shock US. In the first phase of the study, excitatory classical conditioning produced a decelerative HR CR to a CS+ that was consistently paired with the US. Subsequently, a subgroup of the same rats demonstrated an accelerative HR response to a different CS (CS-) that was given in an explicitly unpaired relation to the US. A second subgroup receiving truly random presentations of the CS-and US showed near-zero responding to CS-. Later, the explicitly unpaired group exhibited retarded development of a decelerative HR CR to CS-when CS-was paired with the US during reversal conditioning. In keeping with contemporary treatments of conditioned inhibition (Wagner & Rescorla, 1972), Cunningham et a1. suggested that the opposing directions of the HR responses of the explicitly unpaired group to CS+ and CS-, and the impaired performance shown by this group during reversal conditioning, provided evidence that the explicitly unpaired CS-was inhibitory.Although HR offers a naturally occurring abovezero-baseline level of activity for evaluating the capacities of excitatory and inhibitory stimuli to elicit opposite responses, HR direction in inhibitory paradigms other than the explicitly unpaired arrangement

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