Recent studies suggest that many immune genes, such as those in the major histocompatibility complex (MHC) first identified in the immune system, are also expressed in the developing and adult central nervous system (CNS).They have critical roles in the establishment, function and modification of synaptic connections.Although the MHC class Ⅰ molecules have been found to be expressed in the CNS of different species, a necessary step to elucidate the temporal and spatial expression patterns of MHC class Ⅰ molecules in the brain development has never been taken.Frozen sections were made from the brains of embryonic and postnatal C57BL/6J mice, and the expression of H-2Db mRNA was examined by in situ hybridization.Immunofluorescence was also performed to define the cell types that expressed H2-Db in P15 mice.At E10.5, the earliest stage we examined, H2-Db was expressed in neuroepithelium of the brain vesicles.From E 12.5 to P0,H2-Ds expression was mainly located at cerebral cortex, neuroepithelium of the lateral ventricle, neuroepithelium of aquaeductus and developing cerebellum.From P4 to adult, H2-Db mRNA was detected at olfactory bulb, hippocampus, cerebellum and some nerve nuclei, such as the nucleus of oculomotor nerve (ON), the vestibular nucleus (VN), the facial nucleus (FN) and the cochlear nucleus (CN).In these nuclei, the staining signal of H2-Db began to appear at P4, reached a peak at P15, then decreased gradually and completely disappeared at adulthood.The major cell types expressing H-2Db in P15 hippocampus, cerebral cortex and olfactory bulb were neuron.H2-Kb signal paralleled that of H2-Db and the expression levels of the two molecules were comparable throughout the brain.The investigation of the expression pattern of MHC class Ⅰ molecules at both embryonic and postnatal stages is important for further understanding the physiological and pathological role of MHC class Ⅰ molecules in the developing CNS.