In eight patients with idiopathic Parkinson's disease, loss of nerve cells from the locus caeruleus, together with reductions in capacity for function in those of the dorsal motor vagus nucleus, the supraoptic and paraventricular nuclei of the hypothalamus and the nucleus basalis of Meynert in the substantia innominata are reported. These changes were more severe in three of these eight patients where mental impairment was present, though in none of these three was the number of senile plaques and neurofibrillary tangles or extent of granulovacuolar degeneration greater in the hippocampus and other cerebral cortical areas than would be expected for the age of the patient. Although these same three patients had received long term L-dopa, it seems that their pathology was related to the disease process and was not drug induced. It is possible therefore, that the dementia seen in some patients with Parkinson's disease is related to degeneration and failing function of noradrenergic and cholinergic neurotransmitter systems and not always associated with the coincidental acquisition of Alzheimer type lesions (Alzheimer's disease).