The multiunit activity in the medial septal-diagonal band complex (MSDB) and field potentials of the hippocampus were simultaneously recorded in waking healthy rabbits (control) and in the same animals that were then exposed to kindling stimulation of the perforant path. In the control, the bursts of spikes in one group of rhythmic MSDB neurons phase-locked to the top of theta waves in the hippocampus, and in the second group, to the trough of these waves. The stimulation evoked seizure afterdischarges in the hippocampus and seizure bursts of spikes separated by periods of inhibition in MSDB neurons. In the first group of septal cells, seizure bursts coincided with inhibitory periods between afterdischarges in the hippocampus; in the second group, these bursts were observed during seizure afterdischarges, suggesting that different MSDB cells play opposite roles in the development of seizures. Evoked afterdischarges were spontaneously followed by recurrent ictal events; neuronal oscillations at the theta (6-7 Hz) or "twice-theta" frequency (12-14 Hz) preceded these secondary epileptic discharges. As a result of kindling, interictal spikes were recorded in the hippocampus; at the same time, synchronous bursts of many cells appeared in the MSDB. In the epileptic brain, the frequency of both the hippocampal theta rhythm and MSDB neuronal theta bursts increased; in the septum, an augmentation of neuronal rhythmicity was also observed. Theta oscillations, either spontaneous or evoked by sensory stimulation, abolished the epileptiform events. Evidently, the activities within the theta range during preictal and interictal periods are of different significance in the generation of seizures.