In order to study the pathogenesis of stress-related mental disorders, Nissl staining,Golgi staining and electron-microscope were used to explore the effects of chronic stress on morphology and structure of hippocampal CA1 and CA3 pyramidal neurons in rats.The results showed that no changes of morphology and structure were observed in hippocampal CA1 pyramidal neurons in stress group. The number of hippocampal CA3 pyramidal neuron of stress group（35.14±3.85）was significantly less than that of control group（38.74±3.54），P＜0.05. The total length of apical dendrite of stress group（155.67 μm±33.32 μm）was significantly shorter than that of control group（195.63 μm±34.61 μm）,P＜0.05. In addition, the ultrastructural changes,including the condensation of the cytoplasm,the reduction of the cell volume,the shrinkage of the nucleus membrane,the degeneration of the mitochodria, and the illegibility of the rough endoplasmic reticulum, were observed in hippocampal CA3 pyramidal neurons in chronic stress rats. It is suggested that the changes of morphology and ultra structure of pyramidal neurons in hippocampal CA3, not in CA1,would underlie the pathophysiology of the stress-related mental disorders.