Density and distribution of senile plaques, neurofibrillary tangles and neuronal loss in various sites within the hippocampal system were examined at autopsy in the brains of 10 patients with Alzheimer's disease and in 5 control brains. The presence of neurofibrillary changes was found mostly in layer II of mid-olfactory cortex and in CAI sector of the hippocampus, while the largest number of senile plaques was noted in deep layers of the mid-olfactory cortex and in the molecular layer of the dentate gyrus. Morphometric examination indicated the presence of pathological changes both in the structures of origin of the main hippocampal tracts as well as in those of their termination. However, neurofibrillary pathology turned out to predominate in sites of penetrating tracts departure, while in areas in which they terminate senile plaques prevailed. Observed pathological changes lead to a dramatic loss of nerve cells, most marked in the mid-olfactory cortex (56%) and in CAI sector (46%). Neuronal loss results in damage of main afferent tracts conveying information to the hippocampus from various parts of the central nervous system.