The AChE fiber distribution within the human hippocampus and parahippocampal gyrus was studied in order to provide normative data for the examination of cholinergic fiberarchitecture in human pathology and to clarify the cytoarchitectonic organization of these structures. A modification of the Koelle method was used to stain temporal lobe serial sections from 6 neurologically normal human brains collected at autopsy. The hippocampal formation contains some of the densest staining of any cortical area. Regions with the heaviest concentrations of AChE fibers include a thin band along the inner edge of the molecular layer of the dentate gyrus (ml-DG) and parts of the CA2, CA3, and CA4 sectors of Ammon's horn. Staining is of intermediate intensity in the CA1 region. The subiculum (S) is more lightly stained than the CA fields. Staining in the parahippocampal gyrus is generally less dense than in the hippocampal formation. The most conspicuous feature of the human entorhinal cortex (EC) is the AChE-rich fiber patches seen overlapping the stellate cell islands in layer II. An additional band of relatively dense AChE staining is identified in layers IV-V. Prominent AChE-rich polymorphic neurons are present within the hilum of the dentate gyrus. The CA1/subiculum transition in Nissl preparation is characterized by an oblique interdigitation of CA1 cells. The transition from EC to prorhinal cortex occurs along the medial bank of the rhinal sulcus and is characterized by a band of AChE staining, which slopes obliquely away from layer II until it joins an intermediate pyramidal cell layer. Some comparisons with AChE staining in the monkey were made. The monkey has a similar pattern except in DG, where the intensely AChE staining band along the inner ml-DG is thicker and much more prominent. In comparison to the human, the monkey has more conspicuous AChE staining in the parasubicular region.