Thymosin beta 4 is a protein expressed in most rodent and human tissues, including the brain, and is thought to participate in neurite outgrowth during development by sequestration of G-actin necessary for growth cone extension. Under normal conditions in the adult rat brain, the gene has been suggested to be expressed in microglia and CA1-CA2 hippocampal pyramidal cells. Here we show by using in situ hybridization that t beta 4 is dramatically up-regulated in the infarcted brain area after focal ischaemia. In addition, moderate t beta 4 induction is detected in the perifocal zone, substantia nigra and CA1-CA3 hippocampal pyramidal neurones. Macrophages are probably responsible for t beta 4 induction in the infarcted region, and activated non-neuronal cells probably contribute to the induction seen in the thalamus, substantia nigra and perifocal zone. However, the pyramidal neurones in the hippocampus show t beta 4, upregulation which may be related to restoration of neurite circuits after focal ischemic damage.