Distribution of adult T cell leukemia derived factor (ADF/TRX), a human thioredoxin homologue, was studied by immunohistochemistry in gerbil brain during reperfusion after transient cerebral ischemia. In control brains, immunoreactivity was observed widely in the central nervous system, including the ependyma, tanycytes, endothelial cells as well as subcommisural organs, and weakly in the neuronal cell bodies. During reperfusion, ADF/TRX was expressed in glial cells in the CA1 and dentate hilus of the hippocampus, and in a few cases in the lateral portion of the caudateputamen. ADF/TRX positive glias first appeared after reperfusion for 24 h, and the intensity of staining peaked at 72 h and diminished after 7 days of reperfusion. They were concentrated around microvessels and identified to be astroglia by double labeling immunohistochemistry with glial fibrillary acidic protein as marker. Immunoblotting analysis demonstrated an increase of ADF/TRX in the postischemic hippocampus. Astroglial expression of ADF/TRX in postischemic injuries suggests a role in neuroprotection by hydrogen peroxide reducing and protein-refolding activities or in modulation of local immune responses.