Objective To study effect of chronic arsenism on ultra-structure of rats’ hippocampus CA3. Methods 50 SD rats were randomly divided into 2 groups(25 rats in each group):the control group and the arsenic poisoning group. The control group drank distilled water. The arsenic poisoning group drank distilled water containing 100 mg/L AS2O3. Both groups were fed on common feed. All rats were killed after 4 months and the hippocampus tissue was observed by optic and electron microscope. Results By optic microscope,the pyramidal cells at the hippocampus CA3 region of the control group were dense and orderly. The cells bodies were pyramidal and clear,cytoplasm were abundant with Nissl bodies. While the pyramidal cells of the arsenic poisoning group were fewer and scattered unorderly and the pyramidal cell form was irregular. Nissl bodies in cytopalsm were fewer or missing.The arsenic poisoning group's cells were fewer than the control group’s (P