The serotonin (5-HT)-containing median raphe nucleus has been shown to be critically involved in the control of desynchronized (non theta) states of the hippocampal electroencephalogram (EEG). We examined the activity of 181 cells of the median raphe nucleus in the urethane-anesthetized rat and found that approximately 80% (145/181) of them showed changes in activity associated with changes in the hippocampal EEG. These cells were subdivided into theta-on (68%) and theta-off (32%) based on increased or decreased rates of activity with theta, respectively. They were further classified as slow-firing (~1 Hz), moderate-firing (5-11 Hz), or fast-firing (>12 Hz) theta-on or theta-off cells. The slow-firing cells as well as a subset of moderate-firing theta-off cells displayed characteristics of "classic" serotonin-containing raphe neurons. All fast-firing neurons were theta-on cells and showed either tonic or phasic (rhythmical) increases in activity with theta. We propose that: (1) the slow-firing cells (on and off) as well as a subset of moderate-firing theta-off cells are serotonergic neurons; (2) the phasic and tonic fast-firing theta-on cells are GABAergic cells; and (3) these populations of cells mutually interact in the modulation of the hippocampal EEG. An activation of local serotonergic and GABAergic theta-on cells would inhibit 5-HT slow- or moderate-firing theta-off projection cells to release or generate theta, whereas the suppression of serotonergic- or GABAergic theta-on cells would disinhibit 5-HT theta-off cells, resulting in a blockade of theta or a desynchronization of the hippocampal EEG. A role for the median raphe nucleus in memory-associated functions of the hippocampus is discussed.