The report of "time cell" revealed that rodent hippocampal neurons contribute to time perception. However, the mechanism of time perception has not been unveiled enough, because the process of time perception is abstract and complex. In this study, we investigated the function of rat hippocampus in duration discrimination of seconds. We recorded multi-neuronal activities and local field potential (LFP) from the hippocampus of rats while they were performing the duration discrimination task. In our task for it, the rats were required to discriminate time intervals of 1 and 3 seconds. We found that most hippocampal neurons did not show any task-related changes in firing rate. On the other hand, theta oscillations (4-9Hz) of LFP showed increase of their power during the period for time discrimination. These results suggest that not hippocampal neuron's spikes but LFP's theta oscillations play an important role in the perception of time of seconds in which working memory is not involved. There is a possibility that rodent's hippocampus has another mechanism of time perception with rhythmic activity of neuronal populations in addition to that with spiking activity of time cells. This mechanism may contribute to perception of various orders of time by interactions with other brain areas, such as striatum and prefrontal cortex.