The firing of lateral septal neurons was examined in relation to the hippocampal theta rhythm in urethane anesthetized rats. In general, the firing rates of these cells were low during both theta and non-theta EEG states. There was no significant change in firing rate between the two states (theta: 8.5 +/- 9.9 spks/sec; non-theta: 6.0 +/- 5.3). Sixty-four of 68 cells fired simple spikes and 4 cells were found to fire bursts of action potentials (complex-spikes). Approximately 30% (21/65) of the cells showed a significant phase relation to the hippocampal theta rhythm. The preferred phases of firing of these 21 cells were broadly distributed. The possibility that the phase-locked firing of LSN cells is due to the phase-locked firing of hippocampal projection cells is discussed.