Microcebus cerebral cortex has been investigated using various technics: amyloid beta-A4 protein, alpha-1 antichymotrypsin and ubiquitin immunocytochemistry, thioflavin S staining for amyloid deposition and Bodian's silver impregnation not only for amyloid plaques but also for degenerated neurites; glial fibrillary acidic protein (GFAP) immunocytochemistry has been also used for astrocytes. Parenchymal beta-A4 amyloid is observed to be deposited in some aged Microcebus cerebral cortex in three forms: 1) diffuse plaques of about 100 microns in diameter; 2) dense plaques with compact amyloid core which are thioflavin-positive; these two types of amyloid plaques (AP) are also stained both with the antibodies against alpha 1-antichymotrypsin and ubiquitin respectively; they are also associated with GFAP-immunoreactive astrocytes; 3) extensive ribbon-like infiltrations enclosing multiple blood vessels. The distribution of the AP using an automatic image analyser has showed that the density of the AP in the neocortex fluctuates among the different cortical regions. The density is respectively up to 16 AP/mm2 in the superior part of the temporal lobe and 12 AP/mm2 in the amygdala, 9 AP/mm2 in the prefrontal area of the frontal lobe and 11 AP/mm2 in the central area, 10 AP/mm2 in the parietal lobe and 5 AP/mm2 in the occipital lobe. The compact AP are more often observed in the temporal lobe. In Alzheimer's disease the AP are also made of amyloid beta-A4 protein, alpha 1-antichymotrypsin and ubiquitin. As in Microcebus, in Alzheimer's disease two types of AP are observed: the diffuse and the compact AP; the latter are also thioflavin S positive. The dense AP are not so numerous as the diffuse AP and the density of the AP is greater in the temporal lobe. All these results lead to the conclusion that the AP occurred in age Microcebus brain are similar in many respects to those found in Alzheimer's disease. So, Microcebus murinus, lemurian primate of small size (about 100 g) and relatively short life span (up to 12 years), should provide a valid animal model for investigation of some principal features of Alzheimer's disease.