Single unit activity was recorded from the dorsolateral prefrontal cortex and the anterior cingulate cortex while monkeys were performing a modified differential reinforcement of long latencies (DRLL) task. A total of 252 prefrontal units and 218 anterior cingulate units showed an obvious change in discharge rate (increase or decrease) in association with one or more of the events of a DRLL task. Related units were classified into 3 main groups: S--R event units, reward-error units, and timing units. S--R event units consisted of three subtypes: stimulus-related, response-related, and stimulus--response-related units. Reward-error units contained reward-related units and error-recognition units. Error-recognition units showed a vigorous increase in firing only after incorrect responses. These units were also responsive to omission of reinforcement on correct trials. Three types of timing units were distinguishable. The first one showed an anticipatory change prior to stimulus onset, and the second one exhibited a gradual anticipatory change preceding the time of responding. The third one manifested a sustained change during delay and an abrupt cessation of change in firing at the time of response initiation.