A groggy rat is a single autosomal recessive mutant displaying a movement disorder. Using Gallyas-Braak (G-B) staining of sections of brain and spinal cord from groggy and Slc:Wistar rats, argyrophilic neurons were seen in some regions of the 180-day-old groggy rats. The numbers of these neurons and of the regions exhibiting these neurons in groggy rats increased with age. In 730-day-old groggy rats, these neurons were especially numerous in the red nucleus, reticulotegmental nucleus of pons, intertrigeminal nucleus, facial nucleus, all the reticular nuclei of medulla, hypoglossal nucleus, and spinal cord layers 7–9. Using electron microscopy, silver grains after G-B staining and immunodeposits after staining with an anti-phosphorylated tau antibody, AT8, showed the same localization in the lipofuscin granules in the neurons of facial and hypoglossal nuclei of 730-day-old groggy rats. However, AT8 immunoreactivity was found not only in the lipofuscin granules of the facial and hypoglossal nuclei of the aged-matched Slc:Wistar rat, but also in those of the G- B-negative cerebellar nuclei of groggy and Slc:Wistar rats. These facts suggest that the AT8-immunopositive tau in the argyrophilic neurons of aged groggy rats is modified to react with the G-B staining.