Evidence has accumulated suggesting that the median raphe (MR) mediates hippocampal theta desynchronization. However, few studies have studied theta related neural circuitry during MR manipulation. In urethane anaesthetized rats, we investigated the effects of MR stimulation on hippocampal field and cell activity using high frequency (100Hz), theta burst (TBS), and slow frequency electrical stimulation (0.5Hz). We demonstrated that high frequency stimulation of the MR did not elicit deactivated patterns in the forebrain, but rather elicited low voltage activity in the neocortex and small amplitude irregular activity (SIA) in the hippocampus. Both hippocampal phasic theta- on and off cells were inhibited by high frequency MR stimulation. MR stimulation failed to affect cells that had neither state or phase relationships with theta field activity. TBS of the MR induced theta field activity; phase locked to the stimulation. Slow frequency stimulation elicited a state dependent reset of theta phase through a short latency inhibition (5ms) in phasic theta-on cells. Subpopulations of phasic theta on cells responded in either oscillatory or non oscillatory patterns to MR pulses, depending on their intraburst interval. Off cells exhibited a state dependent modulation of cell firing occurring preferentially during non-theta. The magnitude of MR induced reset varied as a function of the phase of the theta oscillation when the pulse was administered. Therefore, high frequency stimulation of the MR appears to disrupt hippocampal theta through a state-dependent, short latency inhibition of rhythmic cell populations in the hippocampus functioning to switch theta oscillations to an activated SIA field state.