Previous observations have suggested that a reactive astrocytic response to damage does not occur in fetal brain. In this study the time course of the astrocytic response to injury in fetal and neonatal rat brains has been assessed using the immunoperoxidase technique for glial fibrillary acidic protein (GFAP). Cold lesions were induced in utero to the forebrain and brain stem of rat fetuses at 16-18 days of gestation. The inflammatory response and the presence of GFAP in the processes of reactive astrocytes were studied in the brains of animals killed from 4 days (20-22 days of fetal life) to 12 days (9 days of post natal life) after the injury. Reactive astrocytes containing GFAP were present at the site of injury in all fetal and neonatal rat brains. Astrocyte processes were thin and short but stained strongly for GFAP. There was a greater amount of astrocytic scar tissue in animals killed 12 days after injury than in those killed after 4 days. In contrast to adults, little mesenchymal component was observed in newly formed scar tissue on the meningeal surfaces of the fetal and newborn rat brain.