In order to analyse how hippocampal activity is related to memory, the activity of single hippocampal neurons was recorded while macaques performed a recognition memory task. In the task, the first time a stimulus was shown, no reward could be obtained, and the second time a visual stimulus was shown, the monkeys could lick to obtain fruit juice. Many other stimuli could intervene between the novel and familiar presentations of each stimulus. Of 660 neurons analysed, 15 (2.3%) responded differently to novel and to familiar stimuli, with the majority of these responding more to novel than to familiar stimuli. The latencies of the differential responses of the neurons were typically in the range 140-260 ms. The responses of these neurons reflected whether a visual stimulus had been seen recently, in that the neurons responded differently to novel and familiar presentations of a stimulus when a median of 21 other stimuli intervened between the novel and familiar presentations. The responses of these neurons were shown to be related to whether the stimuli had been seen before, not to the reinforcement or the lick responses made, in that the neurons did not have comparable responses in a visual discrimination task in which licks were made to a rewarding stimulus but not to another stimulus. It is concluded that the activity of a small but significant proportion of hippocampal neurons is related to whether a stimulus has been seen before recently, and that this processing is likely to be involved in memory.