Activator of G protein signaling 3 (AGS3) is a guanine nucleotide dissociation inhibitor (GDI) to heterotrimeric G proteins of the Galphai/o class. Previous studies have described the tissue distribution and expression changes of AGS3 in brain extracts; however the precise localization of AGS3 in intact tissue has not yet been determined. The aim of the present study is therefore to map in detail the expression of AGS3 mRNA in the rat brain by using in situ hybridization (ISH) histochemistry with a (33)P-labeled riboprobe. Hybridized sections were analyzed at the regional level after exposure to autoradiography films and at the cellular level after coating with a photographic emulsion. Our results confirm a broad distribution for AGS3 which is expressed throughout the brain. Although most regions show a low level of expression, our results reveal a high abundance of AGS3 mRNA in the cerebellum as well as in regions important for neural precursor cell proliferation (subventricular zone of the anterior lateral ventricle), migration (rostral migratory stream) and differentiation (olfactory bulb). In particular, the high abundance of AGS3 in the subventricular zone-olfactory bulb system further documents the potential role of AGS3 in neural precursor cell division, migration and/or differentiation.