A histochemical method for acetylcholinesterase was used to assess the regional density of acetylcholinesterase-rich (putatively cholinergic) axons in the cerebral cortex. A dense plexus of these fibers was observed in all cortical areas. The entorhinal, cingulate and inferotemporal regions were used for quantitative analysis. The paralimbic cortical areas (entorhinal and cingulate) contained a higher density of acetylcholinesterase-rich fibers than the association cortex of inferotemporal area TE. The more superficial layers in all three regions contained a denser net of these fibers than did the deep layers. Aging was accompanied by a modest loss in the density of acetylcholinesterase-positive fibers in the entorhinal and inferotemporal cortex but not in the cingulate area. In brains from patients with Alzheimer's disease, a dramatic loss of these fibers was observed in all three areas examined but the loss was relatively less pronounced in the cingulate area. The results demonstrate that the cholinergic innervation of the cerebral cortex in the human brain displays considerable regional and laminar variations. Regional variations were also observed in the alterations that the cortical cholinergic fibers displayed as a result of normal aging and Alzheimer's disease. The age-related reduction observed in the density of cholinergic fibers in the cerebral cortex was quite modest when compared with the dramatic loss in Alzheimer's disease.