The effect of intravenous atropine (1 mg) or saline on mnemonic function was tested in patients with various forms of dementia and age-matched controls. Examinees consisted of 12 subjects having Alzheimer's disease (AD), multi-infarct dementia (MID), Parkinson's disease without dementia or with dementia, and normal aged controls. Wechsler's adult memory scale was used. Orientation, visual reproduction, and associative learning were not affected by this small dose of atropine in any of the subgroups. On the other hand, digit span and logical memory were selectively impaired by atropine in AD. The more sensitive parameters were the improvement on retesting (reflecting learning abilities). This study suggests that cholinergic dysfunction is responsible for abnormalities in logical memory and digit span in AD or the dementia of Parkinson's disease but not in MID.