A correlation was found among the degree of memory loss, intellectual impairment, the quantity of senile plaques, and a decrease in choline acetyltransferase and acetylcholinesterase activity in patients affected by senile dementia of the Alzheimer type. In this study, patients were subjected to a series of computerized electroencephalographic (EEG) recordings and neuropsychological evaluations after acute administration of the following drugs: placebo; the cholinergic drugs physostigmine, pyridostigmine bromide, and edrophonium chloride; and the anticholinergic drugs scopolamine and orphenadrine. The results obtained show that the acute administration of some cholinergic drugs improved memory and attention performances, whereas the anticholinergic drugs induced opposite effects. The cholinergic drugs exhibited a tendency to shift the EEG spectrum analysis into more normal patterns compatible with the patient's age. This study supports the view that the cholinergic system plays an important role in memory and attention disturbances found in senile dementia of the Alzheimer type.