Background: Failure to pass the blood-brain barrier is a serious challenge to the use of magnesium in the treatment of neurologic disorders. But , recently, applying magnesium oxide nanoparticles that is capable of crossing biological barriers has created new hopes. According to the reinforcing effects of magnesium oxide nanoparticles on memory and ambiguity in the best time of application and duration of its effects, the aim of the present study was to compare effects of pre-and post-training administration of different doses of magnesium oxide nanoparticles on long-term memory.

  Materials and Methods: In this experimental study, fifty- six adult male NMRI mice in the control group and receiving magnesium oxide nanoparticles group at doses of 1, 2.5 and 5 mg/kg (I.P) before and after training were used. Long-term memory of mice in a week on days 1,3and7 days after training (shock) by using step-down device and passive avoidance learning method was assessed. Time latency in coming down the secure platform was considered as the memory assessment scale.

  Results: The results showed that injection of nano-magnesium oxide in both the 2.5 and 5 mg/kg improved memory through increasing the latency in coming down the secure platform during a week (p< 0.001), without any changes in locomotor activity, whereas, it had no effect at 1 mg dose. Pre-training injection of magnesium oxide nanoparticles increased memory relatively, it wasn't statistically significant compared to the control group.

  Conclusion: According to the above results, magnesium oxide nanoprticles improves long term memory, and it is possible when the training and the acquisition has occurred, otherwise it will not make a significant impact .