As an alternative approach to the study of hippocampal function in relation to behavior, the averaged evoked potentials (AEPs) evoked by electrical stimulation of the Schaffer collaterals (SCH), the alveus and the contralateral hippocampus were recorded at various depths in the hippocampal CA1 region of freely moving rats with chronically implanted electrodes. Significant correlations between AEPs, behavior and EEG were found. At one end of the continuum of AEPs were those recorded during large irregular activity (LIA), an EEG pattern associated with slow-wave sleep or awake-immobility. These AEPs had large early peak and low-amplitude late peaks. At the order end of the continuum, during high frequency theta EEG associated with behaviors such as walking, postural change or phasic paradoxical sleep. AEPs had a smaller early peak, increased later peak(s) and appeared oscillatory. The evoked population spike, a synchronous postsynaptic firing of CA1 neurons, was smaller during behaviors associated with theta than during those associated with LIA. It is postulated that a recurrent inhibitory circuit within the hippocampus can account for the change of the AEPs with EEG and behavior and with stimulus intensity. During theta EEG, the negative feedback may increase such that the evoked population excitatory postsynaptic potential and the evoked population spike decrease and ascillatory response is more readily elecited. The excitability state of hippocampal CA1 may be described by the negative feedback gain in this model.