Basic fibroblast growth factor (FGF) mRNA is increased 4 h after cortical brain injury. In situ hybridization reveals that the increased mRNA persists for at least 2 weeks and that, in areas adjacent and ipsilateral to the lesion, the expression of basic FGF mRNA is also modified. As an example, at three days distal from the lesion, mRNA can be detected in ependymal cells of the lateral ventricle and in selected cells of the hippocampus and cortex. Endothelial cells also synthesize basic FGF mRNA. The increase in basic FGF mRNA is paralleled by similar changes in the localization of the basic FGF protein. Both the intensity and number of cells which stain for basic FGF are increased when they are compared to staining in either the contralateral side or to comparable areas of unlesioned brains. The pattern of mRNA expression is similar from 4 hours to 14 days. Early in the response (4 h to 3 days) on the border of the lesion, the presence of basic FGF is most obvious within the MAC-1-immunopositive population (macrophages and/or microglia). From 7 days to 2 weeks, there has been extensive hypertrophy of the reactive astrocytes which stain intensely for anti-basic FGF(1-24). We conclude that there is increased basic FGF as a function of injury to the CNS. In view of the observation that it is an early and persistent response, the possibility that it plays multiple functions in the regenerative capacity of the CNS is discussed.