Abstract A computerized telemetry system was used to monitor heart rate (HR), core temperature (CT), and gross locomotor activity in rats treated with saline or neuropeptides during a passive avoidance behavior task. Rats were exposed to single mild footshock (0.15 mA, for 3 s). Retention tests were conducted at 24 and 48 h after the learning trial. One h prior to the 24-h retention test, each rat received one of the following treatments (SC): saline (SAL), desglycinamide [Arg 8]-vasopressin (DG-AVP), ACTH 4–10, or desglycinamide-oxytocin (DG-OXT), at a dose of 3 μg/rat for DG-AVP and DG-OXT, and 50 μg/rat for ACTH 4–10. Rats treated with SAL showed a modest increase in avoidance latency accompanied by bradycardia at both retention tests. Rats receiving DG-AVP retained the highest avoidance latency among the experimental groups at both the 24- and 48-h retention test. These rats showed a decrease in HR of the same magnitude as the SAL-treated animals at both retention tests. Rats treated with ACTH 4–10 showed an increase in avoidance latency during the 24-h but not during the 48-h retention test. In addition, following ACTH 4–10 treatment, a tachycardiac response was found during the 24-h retention test. DG-OXT induced both behavioral and cardiac responses opposite to those found in rats given DG-AVP. CT gradually increased while the rats remained on the platform, irrespective of the treatment. Changes in HR and CT were not influenced by somatomotor activity, as no difference in gross locomotor activity was found among the groups. As found previously, DG-AVP had a long-term and ACTH 4–10 a short-term effect on passive avoidance behavior. The finding that these peptides have opposite effects on HR, argues for different mechanisms of action—memory, respectively arousal—mediating these responses at both behavioral and autonomic levels. The finding that DG-OXT exerts actions opposite to those of DG-AVP further confirms its proposed role of an endogenous amnesic compound. Thus, it is concluded that neuropeptides differentially affect autonomic responses and behavior in rats.