Wistar rats were given one-trial passive avoidance training followed by ECS or sham ECS. They were repeatedly tested at 24-h intervals for a total of five test sessions of 3 min each. At 24 h, the sham ECS group showed avoidance behavior while the ECS group failed to avoid the shock compartment. The sham ECS group continued to avoid throughout the five testing sessions. The ECS group showed avoidance behavior at 72, 96, and 120 h. There were no significant differences at these times between ECS and sham ECS. Systematic control groups receiving no footshock on noncontingent footshock never developed avoidance behavior. These groups were not different from the ECS group at 24 and 48 h, and were significantly different from the ECS group at 96 and 120 h. Since the effects of ECS were shown to be temporary, the results are taken to indicate that ECS acts on retrievability of memory rather than on memory storage.