Single unit recording of neurons in the orbitofrontal cortex of the alert rhesus monkey was used to investigate responses to sensory stimulation. 32.4% of the neurons had visual responses that had typical latencies of 100-200 ms, and 9.4% responded to gustatory inputs. Most neurons were selective, in that they responded consistently to some stimuli such as foods or aversive objects, but not to others. In a number of cases the neurons responded selectively to particular foods or aversive stimuli. However, this high selectivity could not be explained by simple sensory features of the stimulus, since the responses of some neurons could be readily reversed if the meaning of the stimulus (i.e. whether it was food or aversive) was changed, even though its physical appearance remained identical. Further, some bimodal neurons received convergent visual and gustatory inputs, with matching selectivity for the same stimulus in both modalities, again suggesting that an explanation in terms of simple sensory features is inadequate. Neurons were also studied during the performance of tasks known to be disrupted by orbitofrontal lesions, including a go/no go visual discrimination task and its reversal. 8.6% of neurons had differential responses to the two discriminative stimuli in the task, one of which indicated that reward was available and the other saline. Reversing the meaning of the two stimuli showed that whereas some differential units were closely linked to the sensory features of the stimuli, and some to their behavioural significance, others were conditional, in that they would only respond if a particular stimulus was present, and if it was the one being currently rewarded. Other neurons had activity related to the outcome of the animal's response, with some indicating that reinforcement had been received and others, that an error had been made and that a reversal was required. Thus, neurons in the orbitofrontal cortex possess highly coded information about which stimuli are present, as well as information about the consequences of the animal's own responses. It is suggested that together they may constitute a neuronal mechanism for determining whether particular visual stimuli continue to be associated with reinforcement, as well as providing for the modification of the animal's behavioural responses to such stimuli when those responses are no longer appropriate.