In free-moving male rats chronically implanted with electrodes in the septal area and hippocampal formation it was possible to drive the hippocampal theta rhythm by low-frequency septal stimulation. Measurements of the function relating the threshold driving current to stimulation frequency demonstrated a minimum at an inter-pulse interval of 130 ms (7.7 Hz) over the range 100-170 ms. The observed function was reliably obtained in single animals, showed good inter-observer agreement and good stability over time. and was little affected by the exact stimulation site (medial or lateral septum), recording site (dorsomedial subiculum or hippocampus) or stimulation parameters (pulses or interrupted high-frequency trains) When thresholds for septal elicitation of evoked potentials were similarly estimated there was no minimum at 7.7 Hz. There was also no deviation from linearity at 7.7 Hz when the frequency of the hippocampal theta rhythm was plotted as a function of the intensity of reticular stimulation. Movement accompanied theta rhythm elicited from the reticular formation. but not theta rhythm elicited by septal stimulation.