Place cell activity in the hippocampus constitutes a neural representation of space. The dynamics of the place cell activity for familiar environment changes gradually over time, suggesting that this temporal dynamics enables to allocate different neural codes for spatially identical but temporally different episodes. To understand the mechanisms determining the dynamics of place cell populations, activity of hippocampal CA1 neurons was imaged during repeated performance in a spatial memory task. Comparing ensemble representations among multiple task sessions revealed that overlap rate of active place cell population was time-dependent, but independent of the number of tasks within a fixed time. This time-dependent change of hippocampal ensemble activity was suppressed by the administration of an NMDA receptor antagonist. These results suggested that the gradual change of activity pattern works as a time code, and NMDA receptor-dependent processes forms the code.