The aim of this study was to evaluate the glial fibrillary acidic protein immunoreactive (GFAP-IR) astrocytes in the periaqueductal gray matter (PAG) of rats treated with monosodium glutamate (MSG). This study was performed on PAG sections of rats treated subcutaneously with MSG (2 g/kg b.w. (group I) and 4 g/kg b.w. (group II)) and saline solution (group C). Immunohistochemical reactions were performed by peroxidaseantiperoxidase method using an antibody against GFAP protein. In animals receiving MSG in dorsomedial (dm) and dorsolateral (dl) parts of PAG the majority of astrocytes had GFAP-IR bodies and few thin, poorly branched processes. In lateral (l) and ventrolateral (vl) parts of PAG cells with numerous, branched processes with GFAP expression were observed. Some astrocytes in vl PAG of group II rats were characterized by strongly stained cytoplasmic bodies and thick, multiple branched processes. Statistically significant differences in the number of GFAP-IR astrocytes were shown between control animals and group II in dm PAG and dl PAG. All MSG-treated animals revealed statistically significantly fewer immunonegative structures for GFAP as well as all cells in the PAG. In the PAG of rats receiving MSG, especially at high dose, there is an increase in GFAP immunoreactivity in astrocytes. The hypertrophy of GFAP-IR cells and the probable lack of their proliferation may indicate isomorphic astroglia in response to increased extracellular glutamate concentrations. However, the decrease in their number in dm PAG and dl PAG may be related to MSG cytotoxicity.