Rac3 is a member of Rho family small GTPases which regulate cellular signaling and cytoskeletal dynamics. The RAC3 gene abnormalities have been shown to cause neurodevelopmental disorders with structural brain anomalies, including polymicrogyria/dysgyria, callosal abnormalities, brainstem anomalies, and cerebellar dysplasia. Although this evidence indicates that Rac3 is essential in brain development, not only its molecular mechanism but also the expression profile is yet to be elucidated. In this study, we carried out expression analyses of Rac3 with mouse brain tissues. In immunoblotting, Rac3 exhibited a tissue-dependent expression profile in the young adult mouse and was expressed in a developmental stage-dependent manner in brain. In primary cultured hippocampal neurons, while Rac3 was distributed mainly in the cytoplasm, it was visualized in axon and dendrites with partial localization at synapses, in consistent with the observation in biochemical fractionation analyses. In immunofluorescence analyses with brain slices, Rac3 was distributed strongly and moderately in the axon and cytoplasm, respectively, of cerebral cortex at postnatal day (P) 2 and P18. Similar distribution profile was also observed in hippocampus. Taken together, the results obtained strongly suggest that Rac3 plays an important physiological role in neuronal tissues during corticogenesis, and defects in the Rac3 function induce structural brain anomalies leading to pathogenesis of neurodevelopmental disorders.