Objective Marginal division (MrD) of striatum is a universal structure in the mammalian brain,and it plays an critical role in learning and memory.In the present study,we try to investigate the synaptic ultrastructure of Methionine enkephalin (MET-ENK) immunoreacted fibers connected with neurons in the marginal division of the striatum in the monkey brains to explore the ultrastructural basis of the mechanism of learning and memory function in MrD.Methods Six male monkeys (macaque) were perfused with paraformaldehyde through heart to fix the brain and the brains were sectioned by a cryostat.Sections of the brains were performed immunohistochemical staining to detect the MET-ENK expression in the stfiatum; the areas with positive immumohistochemical staining was performed ultrastructural observation for morphological characteristics of the MET-ENK synapses in the MrD.Results Immunohistochemistry staining showed a dense arrangement of MET-ENK immunoreactive cells between the putamen and globus pallidus.Five major types of MET-ENK synapses were identified in the MrD:the axo-dendritic synapses,the axo-spinous synapses,the axo-somatic synapses,the axo-axonic synapses and the compound synapses.Conclusion The MET-ENK synapses in the MrD are diverse and complex,and can be distinguished from the rest of the striatum.