Extracellular single-unit recording techniques were used to examine the firing characteristics of neurons in the medial septum/diagnol band of Broca complex (MS/DB) following injections of the 5-HT1A agonist, 8-OH-DPAT, into the median raphe nucleus (MRN) of urethane-anesthetized rats. It had previously been shown that MRN injections of 8-OH-DPAT produce hippocampal theta rhythm. Injections of 8-OH-DPAT into the MRN produced a change in firing characteristics of MS/DB neurons from an irregular discharge to a pattern of rhythmical bursting in synchrony with hippocampal theta rhythm. Cross-correlational and coherence analyses demonstrated that the rhythmical firing pattern of MS/DB neurons strongly correlated with rhythmical fluctuations in the hippocampal EEG during periods of hippocampal theta produced by 8-OH-DPAT injections, but not during baseline conditions (i.e. hippocampal desynchronization). The results suggest that MRN control of the hippocampal EEG is modulated by the MS/DB. Serotonergic projections from the MRN to the MS/DB may normally act to inhibit the rhythmical bursting of MS/DB neurons, thereby producing hippocampal desynchronization. Suppression of MRN 5-HT neurons by MRN injections of 8-OH-DPAT would disinhibit MS/DB neurons, allowing them to burst rhythmically and thereby produce hippocampal theta rhythm.