We studied the temporal and spatial distribution of the mRNA encoding for thymosin beta 4 (T beta 4), a small acidic actin-sequestering peptide, during the early postimplantation mouse development. Analysis of total embryo RNA demonstrated a strong activation of T beta 4 gene after gastrulation and coincident with neurulation. In situ hybridization showed that T beta 4 mRNA was strongly expressed in the central nervous system and peripheral ganglia, paralleling the gradient of neuronal differentiation. An intense signal was also observed in intraventricular macrophages and blood vessels. The role of T beta 4 in mammalian neuroembryogenesis is discussed.