Learning requires the ability to adapt to new situations and to respond to new categories of stimuli. This function is crucial in children and in adults and malfunction can lead to a debilitating disorder known as Alzheimers disease or senile dementia. Acetylcholine (ACh) is one of several neuromodulators implicated in the brains adaptive behavior. It has an important role in several cognitive functions including attention, learning, short-term memory and long-term memory. The Nucleus Basalis of Meynert supplies the cholinergic input to the neocortex and shows marked cell loss in Alzheimers patients and in other cognitive disorders (Wernicke-Korsakoff syndrome and Creutzfeldt-Jakob disease). We have examined the effects of scopolamine, an antagonist of muscarinic ACh receptors, on visual object recognition in macaques. Two macaques were taught a categorization task, i.e. to classify stimuli to categories by appropriate behavioral responses. The paradigm involved a task during which an image that belonged to one of the categories was presented. The macaque used levers to categorize the stimulus. Injections of scopolamine disrupted subsequent performance in this task. When the stimuli presented had not been seen before the experiment, scopolamine significantly impaired categorization accuracy. The monkey was still able to carry out the task wit