We studied the acute response of glial fibrillary acidic protein (GFAP) and S100beta gene expression in the cerebral cortex and hippocampus to mild unilateral cortical contusion. Our goal was to evaluate and compare the expression patterns of each gene in the early stages of the astrocytic response to brain injury. RNA was extracted from the cerebral cortex and hippocampus of male rats at 0, 3, 12, 24, or 96 h after lesion or sham-operation, then quantified using an RNase protection assay. Contusion produced a robust elevation in GFAP mRNA by 12 h in both brain regions on the ipsilateral side to the contusion. In the cortex, but not the hippocampus, this elevation was sustained at 96 h. S100beta mRNA levels were elevated bilaterally in lesioned animals at 24 h in both brain regions. However, these data are difficult to interpret because sham mRNA levels decreased with time, making it unclear whether contusion stimulates S100beta gene expression or whether it mitigates the inhibitory effect of sham. We further analyzed the effect of contusion on GFAP and S100beta immunoreactive astrocyte density at 96 h postlesion or postsham by double-label immunocytochemistry. All detectable astrocytes under all conditions were S100beta immunoreactive in both brain regions. Furthermore, all S100beta immunoreactive astrocytes in the lesioned ipsilateral cortex were also GFAP immunoreactive, whereas only about 11% of S100beta positive cells were also GFAP labeled in the contralateral lesioned or the ipsilateral sham cortex. In the hippocampus, all S100beta immunoreactive cells were also GFAP immunoreactive under all conditions. These data correlate with the gene expression data at 96 h, and suggest that, at least in the cortex, resident S100beta-expressing astrocytes produce GFAP at levels that are undetectable by immunocytochemistry until they are activated in response to injury.