Evidence has accumulated showing that slow oscillations can coordinate neural assemblies and play a critical role in cognitive processes. Although recent studies have indicated that the interaction between the hippocampus and the medial prefrontal cortex is crucial for learning and memory, the underlying mechanisms remain unclear. In the present study, we recorded EEG potentials from the hippocampal CA1 area (HPC) and the medial prefrontal cortex (mPFC) simultaneously in rats under urethane anesthesia. To investigate the minimal condition of neural activity, we focused on spontaneous oscillations with no external stimulation. We calculated the EEG power and phase using wavelet analysis. We found two differential modes in the hippocampal EEG; accordingly, we divided the hippocampal conditions into 2 groups (with slow versus fast frequency oscillation; SO and FO; threshold at 3 Hz) using the k-means method. We categorized the data as SO versus FO events when the power in the HPC exceeded the estimated threshold of the power during a session for at least 20 sec. Across entire sessions, SO in the HPC and slow-wave (around 2 Hz) in the mPFC showed a significantly high coherence. The phase difference between the slow oscillations shifted when the hippocampal condition changed from SO to FO events, or vice versa. Furthermore, we observed that, during FO events, two cycles of FO were nested into SO in the HPC and the slow wave in the mPFC. These results suggest that the phase synchronization in slow oscillations between brain regions provides fundamental timing, which may serve to regulate and organize neural processes into different information-processing modes.