The prefrontal cortex (PFC) is known to contribute to memory processes such as encoding representations into long-term-memory (LTM) and retrieving these representations from LTM. However, the details of the PFC's contribution to LTM processes are not well known. To examine the characteristics of the PFC's contribution to LTM processes, we analyzed single-neuron activity while monkeys performed a pair-association (PA) task using 12 pairs of complex visual images. Among 60 neurons with sample-period activity, 32% showed the strongest and second-strongest sample-period activities when a particular pair of stimuli was presented (pair selectivity). In addition, the mean latency of sample-period activity was 138 ms in the PFC, which was longer than that observed in ITC neurons under bottom-up conditions, but shorter than that under top-down conditions. These results indicate that the PFC participates in encoding and retrieving information in LTM processes and that pair-selective sample-period activity might be a candidate for the top-down signal that the PFC provides to the ITC during the retrieval of information from LTM.