Although clinical experience suggests that brain injury in the aged is associated with a poor prognosis, little research has examined this phenomenon at a cellular or molecular level. Unilateral 6-hydroxydopamine lesions of the nigrostriatal system were produced in 6-, 15- or 24-month-old rats. In the deafferented neostriatum, the time-dependent induction of glial fibrillary acidic protein (GFAP) was larger and persisted longer in the aged rats. The response of middle-aged rats was intermediate. In contrast, no induction of S-100 or glutamine synthetase was observed in any age group. In a second series of rats with stab wounds in the neostriatum, there were substantially larger GFAP inductions than after deafferentation, but fewer effects of age. However, in both lesion paradigms, GFAP staining increased in the contralateral striatum of old rats, but not in young rats. These data support and extend our earlier work describing larger GFAP RNA inductions after fornix transections in aged mouse hippocampus. The consistency of this exaggerated glial reactivity in the aged brain after modest injury suggests the following: 1) aged astrocytes are more sensitive to gliotrophic factors released by terminal degeneration, 2) larger quantities of such factors are produced after injury, 3) clearance of these factors is delayed in old rodents, and/or 4) aged astrocytes are less able to terminate GFAP inductions after activation. Given the potential role of inflammatory reactions as pathogenic mechanisms in Alzheimer's dementia, these data suggest that age-related glial hypersensitivity may independently increase the risk for some degenerative diseases.