Aged canines exhibit central neuropathological changes strikingly similar to those seen in patients with Alzheimer's disease. In this study, brain tissue from pure bred beagles raised in a controlled environment were examined for Alzheimer-like pathology. The mean age of the animals was 15.6 years. The incidence of plaques among these 29 dogs was 65.5%. Of the 19 samples that demonstrated Alzheimer-like pathology, 18 were characterized as diffuse and one as neuritic. Plaque density was found to be independent of age. Plaque numbers were highest in the perirhinal cortex and the adjacent temporal cortex. Familial influence on plaque development is supported by congruence within 15 of the 16 litters examined (p < 0.001). In this environmentally controlled group the diffuse plaques were rarely converted to the dense neuritic plaques found in Alzheimer's disease.