Objectives : Alteration of hippocampus was demonstrated in the maternal social separation(MSS) pups, separated from dams on postnatal day(pnd) 14 and placed alone. Therefore, to understand the molecular events involved in the MSS, we have initiated a search for gene profiles that are up or down-regulated in the hippocampus of MSS pups. Methods : Analysis of cDNA microarray was performed by using total RNA extracted from the hippocampus of control and MSS pups on pnd 17. Also, passive-avoidance test was demonstrated on pnd 35. Results : Up-regulation of Nedd4a was observed in the hippocampus of MSS pups. Also, MSS rats showed less elongation of latency in passive avoidance test. Conclusion : We suggest that environmental effects of MSS may be altered the neural and/or glial differentiation and synapse formation-related genes which may lead cognitive alterations in MSS rats.