Adult rats with unilateral aspirative lesions of the fimbria-fornix and the overlying cingulate cortex received implants of 17-day-old fetal hippocampal tissue in the lesion cavity, placed to form a tissue bridge across the cavity. From 6 to 8 months later they were equipped with chronic recording and stimulating electrodes in the transplant and the host brain. The dominant electrical pattern of the graft consisted of irregularly occurring sharp waves (SPW) or EEG spikes of 30-150 ms duration and concurrent synchronous neuronal bursts of large cell populations. SPWs occurred during all behaviors but their frequency was significantly lower during running than during drinking. Population bursts of neurons in the graft could be evoked by stimulating either the ipsilateral host hippocampus or the perforant path. Both complex spike cells and single spike cells could be recorded from the graft. The discharge frequency of single spike cells was considerably higher during running than during drinking or immobility. Some of the single spike cells in the graft fired rhythmically at 6-9 Hz during running and walking, and phase-locked with the rhythmic slow EEG activity (theta, theta-EEG) recorded from the intact host hippocampus. Occasionally field theta-EEG was also present in the graft. These findings suggest that at least a portion of the graft neurons had come under the control of the host brain, and by way of the newly established host-graft connections the activity of some graft neurons was regulated in a near-normal manner.