Growth inhibitory factor (GIF) inhibits survival and neurite formation of cortical neurons in vitro and is found abundantly in the normal human brain. The role of GIF is still obscure, although it is reported to decrease in the brain in Alzheimer's disease. We examined changes in GIF mRNA expression in a rat cortical-ablation model with the aid of an in situ hybridization technique. In sham-operated animals, the GIF mRNA was expressed consistently in the cerebral cortex, hippocampus, and thalamus. One day after cortical ablation of the left somatosensory cortex, the expression tended to decrease in the cortex ipsilateral to the injury. Four days after surgery, it increased markedly in the affected cortex and thereafter returned to the level of the control animals except for the area surrounding the injury, where GIF mRNA again increased 2 to 3 weeks after ablation. The transient increase in GIF mRNA expression may reflect efforts to inhibit excessive sprouting of neurites. We also studied the effect of topically applied basic fibroblast growth factor (bFGF), which has a range of neurotrophic effects, on GIF mRNA expression. Topically applied bFGF enhanced the suppression of GIF at 1 day after surgery, though it did not affect the subsequent response. GIF can therefore be assumed to affect the outgrowth of injured neurites and might play a major role in maintenance of the neuronal network in cooperation with other trophic factors. Modification of these factors may be the key to improve neuronal damage after injury.