Senile plaques were found in the cerebral cortices of three very aged cats (more than 18 years old). The plaques consisted of a coarse assembly of silver staining-positive materials, and was morphologically different from the well-known classical, primitive, and diffuse plaques. Congophilic amyloid angiopathy was observed in a few cortical arterioles of the oldest cat (20 years old). The senile plaques and a few cortical blood vessels were immunopositive for amyloid beta-protein (A beta). A beta-positive materials were also sparsely distributed in the cortical neuropil but did not form senile plaques there. These findings should help to clarify the development of senile plaques and the early stage of A beta deposition.