Id proteins are negative regulators of basic helix-loop-helix transcription factors and are involved in cellular differentiation and proliferation. Four members of the Id gene family exhibit closely related but distinct expression patterns in various mammalian organs of not only embryos but also adults. Among them, Id2 is known to be expressed in Purkinje cells and neurons in the cortical layers of the adult mouse brain, suggesting that Id2 is involved in some neural functions in the adult. To get insight into the role of Id2 in the nervous system, we investigated the localization of Id2 mRNA-expressing cells in the adult mouse brain in detail by in situ hybridization with the radiolabeled antisense probe and compared it with the localization of other Id gene family members. The results indicated that Id2 mRNA is detected in more varied brain regions than previously reported. These regions include the amygdaloid complex, caudate putamen, globus pallidus, substantia nigra pars reticulata, suprachiasmatic nucleus, and the anterior part of the subventricular zone. These results suggest the possibility that Id2 plays a role in the neural activity and cognitive functions. On the other hand, Id1 was barely detectable. Although moderate or low expression of Id3 was observed diffusely, high expression was observed in some specific regions including the molecular layer of the dentate gyrus and the external capsule. Id4 mRNA was detected in the regions such as the caudate putamen and the lateral amygdaloid nucleus. Thus, the expression pattern of Id2 is distinct from those of other Id gene family members.