As part of an effort to characterize Alzheimer's disease-like neuropathy in the canine brain we have determined the age of onset of spontaneous beta-amyloid deposition in 103 laboratory-raised beagles. Tissue samples for each subject were obtained from hippocampal and cortical regions and examined for the incidence and density of beta-amyloid deposition after staining with modified Bielschowsky silver stain and immunohistochemistry. Amyloid deposition was characterized as diffuse plaque or cloud-like formation. The diffuse type of beta-amyloid plaque formation predominated in all brain regions examined. A threshold effect of plaque development was observed; no plaques were apparent in dogs before the age of 10 years, while 36% of dogs aged 11.1-12.9, 60% of dogs aged 13.0-15.0, and 73% of dogs aged 15.1-17.8 developed beta-amyloid deposits. Additionally, a significant increase in plaque density was observed with increasing age.