Alzheimer disease (AD) is a progressive disorder characterized by cognitive and behavioral dysfunction, central to which are deficits in the cholinergic and other neurotransmitter systems. These results in the essential symptoms of dementia, including impairment of memory, judgment, and abstract thinking. The pharmacologic relationships among the various neurotransmitters (e.g., cholinergic, serotonergic, nicotinic, and dopaminergic) are highly complex and are still being investigated. Information on the pharmacologic basis of cognitive and behavioral dysfunction in AD has applications to drug therapy. One method of obtaining this information is by pharmacomodeling, using individual or combined drugs. Joint cholinergic antagonism with both muscarinic and nicotinic blockade combines to produce short-term memory impairment, which approximates to mild AD in normal elderly people. This effect is better than that achieved with either agent alone. Mixed cholinergic and serotonergic antagonism has an effect on the cognitive function of AD patients and on depression-related behavior. Dopaminergic dysfunction is linked with the development of hallucinatory and psychotic symptoms and may also be involved in dysfunction of verbal fluency. Combination pharmacomodeling allows the various behavioral and cognitive deficits in AD to be studied and allows models for drug trials to be developed.