The distributions of senile plaques (SP) and cerebrovascular amyloidosis (CA) were studied by employing thioflavin S and modified Bielschowsky stains, and beta-protein immunohistochemistry on serial sections of the brains of aged dogs older than 10 years. Mature and perivascular plaques, both of which contained compact amyloid deposits, always showed a close topographic relationship to CA. In contrast, the majority of diffuse plaques showed no topographic relationship to CA. Cell bodies of neurons and/or glia were almost always involved in the diffuse plaques. In addition, beta-protein immunohistochemistry demonstrated amyloid deposits on the periphery of occasional neurons. These findings suggest that different mechanisms may be involved in the development of the different subtypes of SP in the brains of aged dogs.