We examined aged and young monkeys using a multiple position reversal task to investigate declines in cognitive functions with aging. The task consisted of an original learning task (simple position discrimination task) and a reversal learning task. While the performance of the aged monkeys was not different from that of the young monkeys in the original learning task, the aged monkeys showed a poorer performance than the young monkeys in the reversal learning task. According to our response analysis, the poor performance of aged monkeys in the reversal learning was not caused mainly by repetition of error responses, but rather by the impairment of understanding of the association between stimulus and reward. These results suggest that the prefrontal cortex, particularly the medial orbital cortex, is impaired with aging. (C) 2002 Elsevier Science B.V. All rights reserved.