Thymosin beta4 is a major actin-sequestering peptide widely distributed in mammalian tissues, including the nervous system. In the present study, we analyse the expression of thymosin beta4 in normal and kainate-treated rat forebrain. In untreated animals, thymosin beta4 messenger RNA is mainly expressed in neurons of the hippocampal formation, neocortex and amygdaloid complex, as well as in oligodendrocytes. Other high-expressing areas are the tanycytic ependyma of the infundibulum, the substantia nigra pars compacta, and the supraoptic and premammillary nuclei. In rats treated with kainate, an excitotoxin that induces synaptic activation in the CA1-CA3 pyramidal neurons of the hippocampus, the levels of thymosin beta4 were clearly increased in the hippocampus and neocortex during the first 2-3 h after injection. In the long term, kainate causes neuronal degeneration in the CA1-CA3 regions of the hippocampus and functionally related structures, provoking a depletion of thymosin beta4 messenger RNA in these areas; however, the levels of this transcript are restored two weeks after kainate injection. Moreover, we have found that, in these degenerating zones, gliosis is accompanied by an elevation of the levels of thymosin beta4 messenger RNA, particularly in the CA1-CA3 region of the hippocampus, the lateral geniculate nucleus and the mammillothalamic tract. The present results demonstrate the existence of relatively high levels of thymosin beta4 messenger RNA in several areas of the rat forebrain, indicating that this peptide plays an important role in the regulation of actin polymerization in these regions of the brain. Moreover, the elevation of this messenger RNA after kainate treatment suggests a function of thymosin beta4 in the production and remodelling of neuronal processes. Finally, our findings provide evidence for a participation of this actin-sequestering molecule in the reactivity of certain types of glial cell that follows kainate lesions.