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This study was undertaken to determine whether a mRNA for glial fibrillary acidic protein (GFAP) was present in increased amounts as a response to injury and, if so, how was its temporal expression related to the demonstration of GFAP by immunocytochemical techniques. A cerebral freeze-injury was produced in mice and at intervals thereafter the animals were anesthetized, perfused with formalin and histological sections of the brain through the injured area were prepared. A riboprobe for GFAP mRNA labeled with S35 and an immunocytochemical probe for GFAP were utilized to localize mRNA and GFAP immunoreactivity, respectively. For mRNA studies, the histological slide exposed to either sense or antisense probe was overlaid with x-ray film or dipped in photographic emulsion. The developed film was quantitated by digital image analysis. Emulsions were examined by dark-field microscopy. The results indicate that mRNA for GFAP is increased in the cortex in the environs of the injury by 6 hours, becomes maximal at 4-5 days, and is present in increased amounts up to 14 days. The message is enhanced in the adjacent cortex, the subpial region, the adjacent corpus callosum and in the ipsilateral and contralateral callosal radiations. This pattern of enhancement follows the distribution of post-injury edema. Glial fibrillary acidic protein is demonstrable at 24-48 hours after injury. Thus, there is a rapid response of the astrocyte to injury with increased mRNA expression that is followed by expression of GFAP immunoreactivity.