A qualitative and quantitative immunohistochemical study of cholinergic systems in the human hippocampal formation was performed with an antibody against choline acetyltransferase. Four control subjects and six patients with Alzheimer's disease, matched for age and post-mortem delay, were examined. Immunoreactive nerve fibres and terminals were visualized, but no cholinergic cell bodies were seen. The distribution of the fibres and terminals suggests that a major afferent cholinergic pathway enters the hippocampus dorsally via the fimbria-fornix, a minor input entering from the temporal lobe along the alvear path. The cholinergic innervation suffers some degenerative change in normal aged subjects, but decreases considerably in density in patients with Alzheimer's disease. The extent of the decrease differs somewhat among the subregions of the hippocampus, but is homogeneously distributed within each subregion, and throughout the rostrocaudal extent of the structure. Compensatory sprouting in reaction to denervation was not detected.