For purposeful behaviors, accurate coding of time is always needed. However, little is known about where in the brain temporal cognition is achieved. To address this issue, we designed a time-reproducing task. In this task, the monkeys had to watch an LED illuminated for 1-7 seconds (cue period). After a delay period, the monkeys had to wait for the same time duration as indicated by the cue (go period) before pressing a button. Following sufficient training, the error rates for the shorter cues (1 or 2sec) were about half of those for the longer cues (≥ 4sec). Neuronal activity was recorded from the medial prefrontal cortex (i.e. area 9) during task performance. Many neurons were activated differentially according to the different time duration of the cue. Such time-selective activities were observed for all of the task periods. The present results suggest that the medial prefrontal cortex may be involved in temporal cognition.