Two experiments compared the capacity of reminder treatments to modify retention performance in rats either strongly trained and given ECS or weakly trained and given no ECS. Heart rate was monitored during the reminder sessions. In Experiment 1, the factorial combinations of stimulation of the mesencephalic reticular formation (MRF) (present, absent) followed by the presentation of the cue originally paired with foots hock in training, a tone (present, absent), were evaluated as reminder stimuli. Results indicated that both groups showed decreased heart rate to the presentation of the tone in the reminder session. However, in the retention test, ECS animals given tone reminder showed increased retention, whereas weakly trained animals showed decreased retention. Experiment 2 used a shorter interval to onset of the tone in the reminder session. Heart rate to the reminder tone again decreased in weakly trained animals, but no change was obtained in ECS animals. No change in test performance as a result of reminder was noted in either group. These results support the notion that ECS following training influences memory retrieval systems and suggest that heartrate reactivity may be used to predict the behavioral effects of reminder treatment in ECS animals.Reminder treatments interpolated between training and a retention test quite consistently increase retention performance in animals given an amnesic treatment following training. In most instances, this reminder effect is not obtained in, for example, animals trained but not given amnesic treatment or animals not trained (e.g., DeVietti & Hopfer, 1974). However, controversy exists as to the meaning of this effect. Some regard the reminder literature as suggesting that amnesic treatments affect memory retrieval processes (e.g.,