Abstract: Objective To investigate NMDAR subunit mRNA expression and ultrastructural changes in hippocampus of rats after exposed to arsenic.Methods Weaned SD rats were divided into four groups randomly and each group had 12 rats.The control group drank tap water,the other three groups drank 2.72 mg/L,13.6 mg/L and 68 mg/L solution of sodium arsenite for 3 months.The arsenic levels of brain were assessed,the ultrastructual changes of rathippocampus were observed with transmission electron microscope,and the expressions of NMDAR subunit mRNA were detected with RT-PCR.Results Arsenic concentration in brain of 1316 mg/L and 68 mg/L groups were significantly higher than that of in control rats(P<0.05).The ultrastructures of hippocampus neuron cells in arsenic exposed rats presented a series of pathologic changes.The level of NR-A mRNA was downregulated.Conclusion Exposed to arsenic can damage neuron cells and vascular endothelial cells in hippocampus of rats,and downregulate the level of NR2A mRNA.