Objective To investigate the changes of somatostatin(SS)immunopositive neurons in the hippocampal formation in Meynert(NBM)-lesioned rat and to make a preliminary analysis of the mechanism of SS participating in learning and memory. Methods In this study, the memory dysfunction model was established by the microinjection of kainic acid (KA) into the bilateral NBM. The passive avoidance task was used to estimate the degree of memory dysfunction. After 1-4 weeks survival, brain sections were stained by immunohistochemical technique to show the SS-immunopositive neurons. Then the SS-immunopositive neurons were counted with an optic microscope. Results Lesions of NBM could remarkably disrupt memory continuance compared with the normal group. There was a highly significant difference between the NBM-lesioned group and the normal group in the number of SS-positive neurons of hippocampal formation(P0.01). There were a few changes of lesion in cellular morphology which present as the slightly illegible cell outline and the shortening or disappeanig processes. Conclusion SS-positive neurons in the hippocampal formation decreased significantly in the NBM-destroyed rat and it may have a role in the memory dysfunction to a certain extent.