Choline acetyltransferase activity (presynaptic cholinergic system) and high affinity binding of cholinergic antagonists (postsynaptic cholinergic system) were measured in brain tissue removed after death from both mentally normal and demented old people. Muscarinic receptor binding sites in frontal cortex decreased with advancing years only in old people without appreciable morphological evidence of senile degeration. Preliminary data for temporal lobe suggested that also in Pick's disease the density of receptor binding sites is reduced. The markers are not significantly reduced in cases of mixed senile and vascular dementia. However, in non-vascular senile dementia of the Alzheimer type, there were indications that the presynaptic marker is selectively depleted. Therefore, centrally acting anticholinesterases might be beneficial, particularly in the early stages of the disease.