Human and animal studies have suggested that exercise has benefits overall health and cognitive function. The aim of this study was to investigate the effect of treadmill running on passive avoidance learning and memory deficit in morphine-treated rats. The passive avoidance learning was measured in different time intervals (1, 2 and 24 h as well as 1 week and 1 month). Four groups of rats were included as follows: control, morphine-treated, exercised-saline and exercised-morphine-treated group. The electrical foot shock and treadmill training (2 h at a speed of 5 m/min for 10 days) were applied for all the groups. The data obtained was analyzed using unpaired Students t-test and ANOVA test with group as the independent variable, and performance in each session (avoidances and crossings) as the dependent variables. The results show that the total time staying in dark box was decreased in exercised-saline and exercised-morphine-treated rats by treadmill running (P<0.05). The avoidance learning was significantly reduced in morphine-treated group as indicated by the increased total time of staying in the dark box compared with the control group (P<0.05). We could conclude that exercise increased the delay time of entry to the dark electrical foot shock box, suggesting that morphine impaired the short-term memory and learning and this was reversed by the treadmill running.