Bethanechol, a muscarinic cholinergic agonist, was injected subcutaneously in eight cases of senile dementia of the Alzheimer type. Simple reaction time was measured before, 15 min and 30 min following the injection. Before injection, the patients had significantly longer reaction time than non-demented controls. A significant shortening of the reaction time was observed 15 min but not 30 min after injection. A second group of eight patients with senile dementia of the Alzheimer type, matched for age and mental impairment were injected with saline. No significant shortening of the reaction time was observed. These results suggest that the reaction time can be shortened by a muscarinic agonist in dementia of the Alzheimer type.