Extracts prepared from prenatal, neonatal, and adult rat brain were examined for the presence of astrocyte mitogens. Extracts were characterized by molecular weight separation on Sephadex G-100. Two major peaks of mitogenic activity, at 80K and 23K daltons, were identified in the adult brain extract. Extract prepared from adult tissue also possessed the lowest specific activity. Extract prepared from neonatal rat brain exhibited four general peaks of activity at approximately 80K, 45K, 23K, and 17K daltons. This extract possessed the highest specific activity. Extract prepared from prenatal rat brain contained six general peaks of activity with unique peaks at 67K, 14K, and less than 10K daltons. In addition, this extract possessed a specific activity intermediate between the adult and postnatal extracts. Differences were also noted among the extracts in their requirement for "permissive" factors found in serum necessary for mitogenic activity. The data have implications for the normal development of astrocytes, as well as for the pathological process of gliosis.