GFAP has long been adopted as the specific marker for pituicyte, a special type of astrocyte. GFAP and S100beta are two commonly used astrocyte markers. Their immunoreactivities differ in different regions of the brain. To our knowledge this issue has not been studied in pituicyte. In our preliminary study, we found that antibodies against GFAP and S100beta stained the pituicytes differently. A detailed investigation with both light and electron microscopic techniques was thus conducted in the rat. At light microscopic level, anti-GFAP and anti-S100beta stained 66.78% and 86.78% of the pituicytes, respectively. It was found at ultrastructural level that this difference was cell type specific. The parenchymatous pituicytes could be stained with antibodies against both GFAP and S100beta, whereas the fibrous pituicytes were only S100beta-immunoreactive. The functional significance of this cell type specificity remains to be elucidated.