The effect of physostigmine on recent memory was evaluated in young and aged rhesus monkeys. All aged monkeys had previously shown impaired memory. The performance of the young monkeys treated with physostigmine was similar to that recently reported for young humans--no effects at low doses, some improvement at a restricted range of doses, and deficits at the highest dose. Although the aged subjects also improved at the same general doses, their overall response as a group was much more variable than that of the younger subjects. The performance of some aged monkeys was impaired by low doses that did not affect young monkeys. Continued improvement was observed in some aged monkeys at the highest dose, which typically impaired young monkeys. These variable effects across aged subjects suggest that physostigmine cannot easily or reliably be used as an agent for treating geriatric cognition. Nevertheless, the differential age-related effects suggest that appropriate manipulation of the cholinergic system may eventually be developed to alleviate some of the cognitive impairments suffered by aged subjects.