In the elderly, loss of brain wet weight and reduction in neuronal populations may be related to declining cognitive function. In some brain areas there appear to be reduced dendritic arborization. In the aging brain, neurotransmitter metabolism is affected. Catecholamine concentration appears to be especially reduced. More marked loss of cells and synapses is seen in senile dementia of the Alzheimer's type, and there is a significant reduction in choline acetyltransferase activity and in the ability of isolated biopsy tissue to synthesise acetylcholine. The significance of these findings and the possibility of drug intervention is discussed.