Alter being trained to avoid/escape a shock by pressing a lever (nondiscriminatedlreeoperantavoidance learning), 30 male rats were assigned to the lollowing groups: 1) "stressalter a period 01 rest": 22 h alter the last acquisition session, rats in this group were subjected to two hours of immobilization stress and then tested. New tests were performed24 and 48 h after the aplication of stress. 2) "stress before a period of rest": in thisgroup, the stressor was administered immediately after the last acquisition sessions. Tesistook place 24, 48 and 72 h. la ter. 3) Control without stress. Results indicate that stressinduced a deficit in the performance of the avoidance/escape response and subsequentin Creases in the number of shocks received. In group 1 this deficit increased with the sub·sequent sessions.