Epileptic seizures occur less during wakefulness or paradoxical sleep, conditions during which hippocampal theta rhythm is seen. This leads to the hypothesis that this rhythm indicates a physiological state of the hippocampal formation which opposes its recruitment into seizures. This was tested by determining the effects of experimental induction or suppression of hippocampal theta activity on seizures. Hippocampal theta activity can be induced by chemical or electrical stimulation of the medial septal nucleus and adjacent nucleus of the diagonal band of Broca. Microinjections of the muscarinic agonist carbachol in the medial septum during pentylenetetrazol (PTZ) induced facial-forelimb seizures stopped behavioral seizures and EEG spiking within five seconds, and caused hippocampal theta activity. Medial septal electrical stimulation at 4-8 Hz had similar effects. Electrolytic medial septal lesions abolished hippocampal theta activity and lowered myoclonic and facial-forelimb PTZ seizure thresholds. Medial septal carbachol injections were also made during electrically kindled limbic status epilepticus. Within ten seconds, ictal behavior stopped and the EEG spike rate decreased by half with a gradual return to the baseline rate over three minutes. These results demonstrate that the hippocampal theta rhythm corresponds to a seizure-resistant condition, providing a possible explanation for the seizure promoting properties of slow wave sleep.