Objective To study the expressions of S100B and glial fibrillary acidic protein (GFAP) in the central nervous system of Sprague_Dawley rats during postnatal development. Methods Twenty_four male SD rats were divided equally into four groups according to different postnatal times:7_day group, 14_day group, 21_day group and adult group.Immunohistochemistry was used to investigate the expressions of S100B protein and GFAP in the brain and the spinal cord. Results The amount and density of S100B positive astrocytes decreased significantly in frontal cortex, hippocampus, striatum, substantia nigra and spinal cord during postnatal development. It seemed that the second through the third week after birth was a critical period for these changes. The amount and density of GFAP positive AST increased gradually in the brain, but it was the opposite in the spinal cord. Double_labeled immunofluorescence of S100B and GFAP in hippocampus CA1 area showed that S100B positive stains were evenly distributed in the pyramidal, polymorphic and molecular layers from the seventh till the twenty first day after birth, but apparently decreased in each layer especially in the molecular layer in adult hippocampus while the immunoreactivities of GFAP increased. The proportion of double labeled cells also increased with the aging and more of them were found in the pyramidal and polymorphic layers.Conclusion Different patterns of the expressions of S100B and GFAP exist during postnatal development. It may reflect the different roles of these proteins on the glial cell development. And it may also indicate that the expressions of S100B and GFAP are regulated by different mechanisms during the course of development, which reflects the differentiation of subpopulations of astrocytes during ontogenesis.