Introduction: The possible involvement of dopamine receptors in the nucleus accumbens (NAc) in nicotine improvement of memory deficit induced by morphine was investigated. The present research describes studies in rats that investigated how morphine administered after successful learning of on inhibitory avoidance task alters the response of the rats, and how these effects are altered when nicotine or dopamine D1 receptor antagonist were infused into the nucleus accumbens (NAc). Materials& Method: Adult male Wistar rats (Pasteur institute, Tehran, Iran) Weighing 200-250 g at the time of surgery were used. All rats were maintained upon their arrival in the laboratory (one week before the experiments) in groups of four per cage, with food and water available ad libitum. Each experimental group consisted of eight animals and each animal was tested once. A step-through inhibitory (passive) avoidance was used for memory assessment in male wistar rat. The results were statistically evaluated by analysis of variance ( one- and two- way ANOVA), in which mean of step- through latencies of the experimental groups on the test day were compared. Further analyses for individual between – groups comparisons were carried out using post hoc Tukey’s test. Statistical significance level of p<0.05 were used throughout. Result: The results showed that Subcutaneous (s.c.) administration of morphine (5 and 7.5 mg/kg) after training decreased step-through latency in the animals when tested 24 h later; indicating impairment of memory induced by morphine. Interestingly, Pre-test administration of the same doses of morphine partly or fully reinstated the decrease of step-through latency induced by post-training administration of the opioid. Moreover, pre- test administration of nicotine (0.2 and 0.4 mg/kg, s.c.) before the test mimicked the effect of morphine on reinstatement of step-through latency. Although Pre-test intra-NAc administration of dopamine D1 receptor antagonist, SCH23390 (0.0
