FERM domain including Rho GEF (FIR) is one of the guanine nucleotide exchange factors for Rac1. FIR, expressed in hippocampal and cortical neurons in vitro, is suggested to be involved in neurite remodeling. We examine developmental regulation of FIR mRNA expression in the mouse brain using in situ hybridization to get insight into its function. FIR mRNA is expressed in the ventricular zone and the intermediate zone as well as the cortical plate and the preplate in the brain from mice during the embryonic stages 12.5 to 14.5. In the brain during the later embryonic stages and the postnatal stages, the expression was restricted to the cortical plate. These results suggest that FIR may play a role not only in neurogenesis, but also in the asymmetrical cell division and migration of neurons.