Significant progress has been made in identifying changes in cholinergic, monoaminergic, and peptidergic neurotransmitter systems in the brains of individuals with Alzheimer's disease. Moreover, neurobiological approaches are beginning to reveal the relationships between these neurotransmitter abnormalities and histological hallmarks of the disease, i.e. neurofibrillary tangles, senile plaques, and loss of specific populations of nerve cells. However, to date, the clinical consequences of these specific neurotransmitter changes remain to be elucidated, the cause of this disorder has not been identified, and no effective therapy is yet available.