The retroactive effects of stress on memory have not received a great deal of empirical attention; however, the research that has been conducted has reported both positive and negative effects of stress on memorial processes. The goal of this study was to examine the effects of a naturalistic stressor—an intense bout of exercise—on memory for inhibitory fear conditioning in rats. Experiment 1 investigated the retroactive effect of a stressful swim treatment on memory for passive avoidance (PA) training. Rats that received an immediate posttraining swim treatment demonstrated a significant enhancement in performance when tested for retention 24 h later. Furthermore, the enhancing effect of the swim treatment was time dependent: Rats receiving the swim treatment 15 min after PA training no longer exhibited reliably better scores than did rats not receiving the swim treatment. Experiment 2 used preexposures to control for the possibility that the swim treatment was enhancing avoidance scores by acting as a punisher rather than a memory modulator. Results indicate that both the group that was preexposed and the group that was not preexposed showed reliably higher scores than did a group of animals receiving only PA training, thus replicating Experiment 1. Furthermore, the results of Experiment 2 suggest that swimming was not simply acting as a punishing agent, since preexposures to the treatment did not attenuate its memory-enhancing properties. The possible role of stress-related hormones on memory processes is considered.