The role of aging in the expression of the astrocyte protein, glial fibrillary acidic protein (GFAP), was examined. In both mice and rats the concentration of GFAP increased throughout the brain as a function of aging. The largest increase (2-fold) was observed in striatum for both species. The neuron-specific proteins, synapsin I and neurofilament-200 (Mr 200 kilodaltons), were not altered by aging in any region of the mouse or rat brain. Brains of aged rats, but not mice, showed a decrease in beta-tubulin. The data suggest that astrocytic hypertrophy observed with aging involves an accumulation of glial filaments.