In this article the effects of neurotransmitter systems or specific drugs on cognitive functions of aged animals and humans are reviewed. While there have been used many different pharmacologic and behavioral approaches to treat and test cognitive deficits in aged animals and geriatric or demented patients, there is still the question of the validity of animal models of human disorders. Attempts are made to show that there are parallels and similarities. These similarities suggest that the neurological and neurochemical changes observed may play common roles in similar behavioral deficits observed in aged animals and humans.