Abstract: In order to investigate whether N-methyl-D-aspartic acid (NMDA) receptor and nitric oxide synthase (NOS) made contributions to the depression induced by chronic unpredicted mild stress (CUMS)，the depression model was established by using the CUMS. Using the stereotaxic and intra-hippocampal microinjection observing rat emotion and behaviors by the change in body weight, sucrose preference test, open field test and forced swimming test, and immunohistochemistry, and the expression of NOS in hippocampus were tested. The results showed that depression-like behavior and the expression of NOS increased in hippocampus induced by rats receiving CUMS for 21 days or intra-hippocampal injections of NMDA receptor agonist. Intra-hippocampal injection of noncompetitive NMDA receptor antagonist MK-801 significantly prevented CUMS-induced depression-like behavioral changes, and decreased the expression of NOS. These results provide evidence that CUMS resulting in development of depression may induce neurotoxicity in hippocampal neurons by stimulating Glu release, inducing excessive activation of NMDA receptor, and increasing expression of NOS.