Specific immunocytochemical methods (AT8) permit evaluation of neuronal changes well before the actual formation of neurofibrillary tangles and neuropil threads. Initial changes are found in the transentorhinal region (temporal lobe). From here the destructive process encroaches upon the entorhinal region, Ammon's horn, and neocortex. Initial changes occur in comparatively young individuals and can also be observed at the same predilection sites in a few species of old aged domestic animals. In a later state of destruction, AT8 immunoreactive neurons develop typical argyrophilic neurofibrillary tangles and neuropil threads. Six stages of disease propagation can be distinguished with respect to the location of the tangle-bearing neurons and the severity of changes (transentorhinal stages I-II: clinically silent cases; limbic stages III-IV: incipient Alzheimer's disease; neocortical stages V-VI: fully developed Alzheimer's disease). Whole mount techniques reveal the lesional pattern of the particularly severely involved superficial entorhinal layer as seen from the free surface of the parahippocampal gyrus. This approach facilitates recognition of even subtle pathologic changes throughout the entire extent of cortical territories such as the transentorhinal and entorhinal regions.