Age-related changes in the canine substantia nigra, were examined using immunostaining for tyrosine hydroxylase (TH), glial fibrillary acidic protein (GFAP), neurofilament (NF), ubiquitin, single stranded DNA (ssDNA), and alpha-synuclein (alphaSN). Brain sections from 34 necropsied dogs, ranging from 2 months to 18 years old, were used for this study. On general histological examinations, several age-related changes, including lipofuscin deposition, polyglucosan bodies, amorphous basophilic inclusions and eosinophilic crystal inclusions, were found in the aged dogs. Immunohistochemically, TH-positive neurons were located only in the substantia nigra. The number of TH-positive neurons was well preserved in all dogs examined, however, the ratio of TH-positive neurons to GFAP-positive glial cells tended to show slight decrease in aged dogs. By ssDNA immunostaining for apoptotic cells, there were no significant results in the number of ssDNA-positive neurons. The number of ubiquitin- and NF-positive swollen neurites was increased markedly in aged dogs. Ubiquitin immunostaining revealed a small number of basophilic and eosinophilic inclusions, although both types of inclusions were negative for NF. By alphaSN immunostaining, no neurons were immunoreactive and no basophilic or eosinophilic intracytoplasmic inclusions were revealed. These results indicate that in the substantia nigra of aged dogs the dopaminergic neurons are well preserved, but intracytoplasmic inclusions and ubiquitin-positive degenerative neurites are commonly found.