To clarify the immunohistochemical features of canine senile plaques (SPs) and cerebral amyloid angiopathy (CAA), the distribution of the amyloid beta protein (A beta) subtypes A beta 40 and A beta 42(43), A beta precursor protein (APP), and glial cell reaction were examined in the brains of seven aged dogs (12-18 years). A beta 42(43) was found to be deposited in all types of SPs, whereas A beta 40 was deposited only in mature (classical and primitive) plaques. CAA, which was located along parenchymal and meningeal arterioles and capillaries, consisted of both subtypes of A beta. APP was exhibited in normal and degenerative neurons and swollen neurites of mature plaques. It was, therefore, considered that A beta 42(43) in diffuse plaques might be derived from APP in neurons, while A beta 40 and A beta 42(43) in mature plaques might be generated from APP in swollen neurites in the plaque. In contrast to the case in humans, in whom deposition of A beta 40 and A beta 42(43) in the mature plaques is predominantly associated with microglial reaction, in dogs we found that it was closely associated with astroglial reaction. The present findings showed characteristics of canine SPs which are different from those of humans.