The immunohistochemical localization of P400/inositol 1,4,5-trisphosphate (InsP3) receptor protein was studied in developing and adult mouse brain by using monoclonal antibodies. The developmental expression pattern of P400/InsP3 receptor protein differed among different classes of neurons. It was first detected in the somata of immature Purkinje cells at embryonic day 17, in the ventrolateral region of the posterior vermis in the cerebellum. Axonal immunoreactivity within the cerebellar nuclei was first present at postnatal day 3. Neurons in the retrosplenial cortex, the anterior olfactory nucleus, and the CA1 region of the hippocampus expressed immunoreactivity earlier than other regions of the brain. In the adult brain, not only the Purkinje cell but also many other types of cells in many areas of the brain expressed P400/InsP3 receptor, though to a lesser extent. These included the neurons in the striatum, globus pallidus, nucleus accumbens septi, anterior olfactory nucleus, olfactory tubercle, precommissural hippocampus, hippocampus, substantia nigra, cerebral cortex, pons, and certain hypothalamic nuclei. Forebrain cortical regions that receive afferents from the olfactory bulb, such as the anterior olfactory nucleus, olfactory tubercle, prepiriform cortex, entorhinal cortex, and amygdala, exhibited distinct immunoreactivity, while olfactory bulb was almost devoid of staining. Immunoreactivity in the axonal pathways was also found in the limbic-hypothalamic pathways, strionigral projection, and part of the corpus callosum. Results of Western blot analysis and 3H-InsP3 binding assay were consistent with the qualitative regional differences of immunoreactivity demonstrated by immunohistochemical study. The location of InsP3 receptor in the brain correlates well with the InsP3 binding sites demonstrated by an autoradiographic study.