Activities relating to the cholinergic system in post-mortem brain tissue have been examined in relation to ageing and Alzheimer-type pathology. As senile plaque numbers increased in non-demented and demented old people, activities of choline acetyltransferase and acetylcholinesterase decreased, butyrylcholinesterase increased and muscarinic receptor binding remained unchanged. The behaviour of these biochemical activities was further examined in relation to the ageing process in mentally normal people. Loss of choline acetyltransferase also occurred, to a lesser extent, with increasing age and muscarinic binding decreased but there was no age-related loss in acetylcholinesterase. These biochemical findings are discussed in relation to the possible involvement of the cholinergic system in 'normal' ageing and in Alzheimer's disease and are compatible with an extension of age-related nerve terminal changes to abnormalities of cholinergic processes in the disease itself.