Objective The aim of this study was to observe the expression of NeuN in the central nervous system (CNS) of adult rats. Methods Normal adult SD rats were perfused and cut into sections on a cryostat. Immunohistochemistry, immunofluorescent and Nissl staining technique were used. Results Four types of NeuN expression in the neurons of CNS were observed: the neurons in most areas throughout the CNS (such as the cerebral cortex, basic ganglia and spinal cord dorsal horn, etc) were found with strong NeuN signals; the neurons in some areas (such as the spinal nucleus of trigemina nerve and the hypothalamic paraventricuar nucleus, etc) revealed middlling signals; the neurons in other nuclei indicated wealer NeuN signals （such as the solitary tract nucleus and the locus coeruleus, etc）;some areas (such as the supraoptic nucleus，the arcuate nucleus and the dorsal nucleus of vagus nerve, etc) appeared devoid of immunoreactivity for NeuN. Conclusion The present results indicate that expressions of NeuN in all the neurons of CNS of adult rats were different, and NeuN even di