Alzheimer's disease and senile dementia are considered primary degenerative disorders of the brain. The delimitation of the two disorders is their onset: whether it is before or after the age of 65 years. As the neuropathological findings in the two disorders are similar they sometimes are sampled together. Biochemical investigations have shown extensive and severe disturbances of neurotransmitters in the brain of patients with Alzheimer's disease and senile dementia. Although the acetylcholinergic system is severely damaged and has been given special etiological importance, it seems that also the monoaminergic, the GABA-ergic and the neuropeptidergic systems are deranged. The extensive damage to the neurotransmitter systems indicates an etiology beyond the transmitter level. It is natural to assume that the disturbed neurotransmitters have pathogenic importance for the disorder. Pharmacological treatment trials have been performed in which the failing systems are substituted. Hitherto, however, these trials have not been as successful as expected.