Age-related increases of the astrocyte marker, glial fibrillary acidic protein (GFAP), were further resolved by in situ hybridization and immunocytochemistry in female C57BL/6J mice. The age groups represented the major stages of reproductive aging: young (5 months), middle-age (18 months), and old (23 and 26 months). GFAP mRNA and protein showed generalized increases in old mice. Major white fiber tracts, such as the corpus callosum, fimbria, stria terminalis, and optic tract, showed increased GFAP immunostaining and mRNA. Gray matter showed robust > or = twofold increases in GFAP mRNA with age, especially in the thalamus and hypothalamus, areas that expressed little GFAP in the young. These generalized age-related increases of GFAP in many brain regions imply the existence of a widespread stimulus for increased activity of astrocytes during aging.