P400 protein, which is identical to the inositol 1,4,5-trisphosphate receptor protein, is a glycoprotein closely associated with the membranes of Purkinje cells. Three types of monoclonal antibodies against P400 protein were employed for the immunohistochemical detection of Purkinje cells in the cerebellum and brainstem of the normal and reeler mouse. Purkinje cells in both types of mice were immunoreactive against anti-P400 antibodies, and the soma, dendrites, axon and even terminal boutons in the cerebellar and vestibular nuclei could be clearly visualized. In the cerebellum of the reeler mutant, the heterotopic Purkinje cells both within and below the granule cell layer were also immunopositive and could be clearly differentiated from the deep cerebellar nuclei, in which neurons were immunonegative. The molecular layer of the reeler cerebellum varied in thickness and certain parts were completely defective. The dendrites within the molecular layer extended from Purkinje cells whose cell bodies were located in the normal position, abnormally in the granule cell layer, or at the surface of the central mass. Outside the cortex of the cerebellum, ectopic Purkinje cells were demonstrated in 3 cerebellar nuclei, the cerebellar medulla and peduncle, and brainstem of the normal and reeler mouse.