: Introduction: Nitric oxide (NO) is a retrograde messenger in hippocampal synaptic plasticity which involves in learning and memory processes. Previous studies revealed that hippocampal pyramidal cells contain NO synthase (NOS) enzyme which produce NO and could be a promising target to evaluate the role of NO in brain cognitive functions. So in this study, using NOS inhibitor (L-NAME), we conducted an experiment to assess the role of NO in passive avoidance learning. Methods: For this purpose, adult male Wistar rats (200-250 gr) were bilaterally implanted into the CA1 region of hippocampus. A week after surgery animals subjected to behavioral tests. 25 min. before training, rats received different doses of L-NAME (5, 10 and 15 μg/0.5μl/side) into the CA1 of hippocampus. Retrieval tests were performed in three different stages after training as working or immediate memory (immediately after training), short-term memory (90 min. after training) and long-term memory (24 h after training). Results: Our finding showed that pre-training injection of 15 μg/0.5μl/side L-NAME significantly increased the number of step-through into dark chamber and decreased step-through latency, for immediate and short-term memory, respectively. Conclusion: These results suggested that hippocampal NOS inhibition impairs both immediate and short-term memory, but have no any significant effect on long-term memory. Thereby hippocampal NO may affect early on learning and memory in passive avoidance task.