Reductions of the levels of transmitter substances and of the activities of enzymes involved in their synthesis have been demonstrated in the aging brain. The sensitivity to the aging process varies for different transmitters and brain regions. Dopamine neurons are more age-sensitive than most other neurons investigated. The metabolism of monoaminergic neurotransmitters is enhanced in the aging brain, as evidenced by increased metabolite/neurotransmitter ratios, perhaps to compensate for the loss of transmitter. In various types of dementia, including Alzheimer's disease (AD) and senile dementia of Alzheimer type (SDAT), several neurotransmitter indices are reduced, as compared to age-matched controls. Moreover, a decrease in neurotransmitter metabolites suggests that compensatory mechanisms are insufficient. No correlation could be found between the neurotransmitter changes and the histological changes characteristic of AD (senile plaques and neurofibrillary tangles). Neither could any relationship between multiple infarctions and neurotransmitter indices be detected. Recently observed changes in the lipid composition of the white matter, indicating demyelinization, in the brains of patients with AD/SDAT, emphasize the multifactorial aspects of dementia. Taken together, the data underline the difficulties in drawing clear demarcation lines between normal and pathological aging and between different subgroups of dementia. Despite the obvious difficulties, future therapeutic efforts should aim at substitution for the neurotransmitter deficiencies. Preventive measures have to await the clarification of the mechanisms underlying neural degeneration. Studies of the toxicity of oxygen and of autoxidation products are among the areas of research that may help to shed light on this problem.