Abstract: In this study,the effect of high Ca2+ level by injecting CaCl2 into bilateral hippocampus on the learning and memory behavior of young mice was investigated,and the synaptic structural parameters in the hippocampal CA3 area were quantitatively analysed.The main results were as follow,compared with control:(1) The STL(step-through latency)of CaCl2 group was shortened in one trial passive avoidance response,and the descrimination learning capacity of CaCl2 group in Y-maze was greatly attenuated (P<0.01),showing the severe impairment of learning and memory in this group.(2) The thickness of post-synaptic density (PSD)of CaCl2 group was thinned significantly (P<0.001),while the width of synaptic cleft of this group was apparently increased (P<0.05).These results suggested that the learning and memory dysfunction caused by over-loaded calcium ions in hippocampus might based on the morphological changes of the synaptic interface stucture.