The study was performed on 19 brains of nondemented patients with age ranging from 36 to 89 years. After embedding in paraffin, coronal 8-microns-thick serial sections were cut and stained either with cresyl violet or with immunocytochemical methods for amyloid and tangles. Morphometrical studies were performed in all parts of the claustrum along its whole extension. Changes related to aging (neuronal loss and decrease in volume) were found in all parts of the claustrum, but the time of origin of these changes is different in various parts of this structure. We did not observe neurofibrillary pathology in any parts of the claustrum. In the oldest subjects a small number of amyloid plaques was found in the paraamygdalar part of the claustrum. We suggest that the neurons of the claustroneocortical loop are affected severely and earlier than those in the claustroentorhinal loop.