The content and polypeptide composition of the glial fibrillary acid protein (GFAP) in the hemispheres' cortex, hippocamp, thalamus and brain stem in the condition of thyroid gland hypofunction in Wistar rats of different age (preadolescent rats five weeks old, pubertal rats five months old and old rats 18 - 20 months old) were investigated. Hypothyroidism was created by adding mercasolil (10 mg/kg) to the standard diet. At the end of experiment the level of thyrotropin and thyroxin in serum was defined with immunoenzyme method. It was established that experimental hypothyroidism caused significant increase 14-72% of the filament fraction GFAP content in the thalamus and brain stem (preadolescent rats) and in the thalamus and hemispheres' cortex (adult rats), Defragmented polypeptides with molecular weight 47 -37 kDa were revealed by immunoblotting. It is connected with the activation of cytosceletal changes of astroglia. There were changes in the group of old rats that is possibly a reason of reduced the flexibility of CNS.