Basic fibroblast growth factor (bFGF) is a polypeptide found within the CNS with potent effects on the survival and proliferation of CNS glia and endothelial cells, and on the survival and outgrowth of CNS neurons. Immunohistochemical methods were used to examine relative changes in the levels and distribution of bFGF following focal brain injury. Two monospecific antisera to bFGF were used to immunostain intact mature rat brain, and brain in which a focal mechanical lesion had been made in the dorsolateral cerebral cortex one week previously. In the intact brain, staining was localized primarily in neuronal cell bodies, especially in limbic structures. In injured brain, a marked increase of bFGF immunoreactivity was found at the borders of lesions, localized to the dense accumulation of cells, many of which resembled 'reactive' astroglia. Such increases in local bFGF concentrations may contribute to the cascade of cellular changes--including glial and capillary proliferation, and neural sprouting--that follows focal brain injury.