Objective To investigate the effect of ulinastatin on hippocampal neuron and matrix metalloproteinase-9 ( MMP-9 ) expression after ischemia-reperfusion injury rats, to reveal the mechanism of protection on hippocampus of ulinastatin.Methods The focal ischemia reperfusion model was made by thread embolism of middle cerebra1 artery, and ulinastatin was used.The animals were killed at 24 h after reperfusion was started.The sections of the brains were processed with Nissl staining and by immunohistochemistry with antibody of MMP-9.Results Corrected optical density (COD) of MMP-9 expression was significantly decreased after administration of ulinastatin.Also ulinastatin can obviously protect the structure of hippocampal neuron.Conclusion Ulinastatin could protect pyramidal cells in CA1 region from damage of ischemia-reperfusion injury and increase the number of normal neurons.Inhibition on MMP-9 expression was involved in the mechanism.