The behavior of 25 dogs was indirectly assessed by a formal questionnaire (evaluation of Age-Related Cognitive and Affective Disorders-ARCAD), filled out by the owner. The density of diffuse and vascular deposits was evaluated using four anti-Aß peptide antibodies, in four temporal areas. Parenchymal, diffuse deposits of Aß42 peptide were found in all aged animals but one. They were Congo red negative and were not immunostained by the anti-Aß40 antibody, contrary to the vascular deposits. The densities of vascular and parenchymal deposits were not correlated. The ARCAD score was correlated with age, density of Aß parenchymal and vascular deposits, and with the number of areas containing deposits (extension index). Multivariate analysis showed that the age and the extension index explained most of the variance. Congo red positivity (indicating that the Aß peptide has the characteristics of an amyloid substance) is limited in the dog to the vascular wall and is associated, as in man, with the deposition of the Aß 1-40 isoform. Parenchymal Aß deposition seems to be a common correlate of behavioral problems in aging dogs.