Introduction: RS67333 is the selective agonist of 5-HT4 and ACPA (Arachidonylcyclopropylamide) is the selective agonist of CB1. The aim of present study was investigate the bilateral injection effects of dorsal hippocampus (CA1) serotonergic system (Agonist of 5-HT4) on fear memoryformation process induced by ACPA in mature male mice. Materials & Methods: 120 Male NMRI mice weighting 23-28 gr were put into control and experimental groups in our experiments. Fear conditioning task was used for conditioning of fear. 24 hours after training, we estimated the fear memory formation. Findings: Intraperitoneal administration of ACPA (0.005, 0.05 and 0.5 mg/kg) causes impairment of fear memory and amnesia. Moreover, intra-CA1 injection of 5-HT4 receptor agonist (RS67333) (0.005, 0.05 and 0.5 µg/mice) in highest dose impaired fear memory and induced amnesia in saline-treated mice, and increased the impairment of fear memory in ACPA-treated mice(P<0.05). Diccussion & Conclusion: Serotonergic system (Agonist of 5-HT4) in the CA1 of hippocampus induced by ACPA interferes in the impairment of fear memory.