In thirteen cases of senile dementia of the Alzheimer type severe loss of nerve cells from the locus caeruleus was frequently seen together with reductions in nucleolar volume and cytoplasmic RNA in surviving cells, averaging 14 and 21 per cent respectively. These histological findings were matched in two cases by biochemical measurements of loss of noradrenaline from all brain regions examined, ranging from 10 per cent in temporal cortex to 53 per cent in hypothalamus. By contrast, neither nucleolar volume nor cytoplasmic RNA was altered in cells of the substantia nigra, nor was dopamine content significantly altered in most regions. In vascular dementia neither noradrenaline nor dopamine metabolism was changed except in regions of local circulatory deficiency. These findings provide evidence of a primary degeneration of the noradrenergic system in Alzheimer type dementia.