Objective To observe the characteristics of preictal hippocampal depth electrode EEG in patients with pharmacoresistant mesial temporal lobe epilepsy,and to accumulate experiences for judgement of epiletic focus and to provide neuroelectrophysiological evidences for surgical resection of epileptic focus.Methods Depth electrode was implanted into bilateral hippocampus of patients with pharmacoresistant mesial temporal lobe epilepsy.Hippocampal depth electrode EEG monitoring lasted for 48 ～72 hours with the patients' AED discontinued,and then the preictal hippocampal EEG data of the 16 patients were analyzed to observe the features of the epileptiform discharges recorded by hippocampal depth electrode.Results 16 patients experienced monitoring for 48 ～72 hours,33 epileptic seizures in 10 patients were detected.The interictal waveform of EEG from each electrode contact was similar to others,Paroxysmal high amplitude and slow wave in 1 patient,paroxysmal fast frequency in 1 patient and spike and slow wave complex in 14 patients on the background were considered to be abnormal electrical activity.Legible EEG waveform was recorded by hippocampal electrode in16 patients during epileptic seizures.The onset of epileptiform discharges manifested with locally original electrical activity with high amplitude and fast frequency in 1 patient(16 Hz) and slow waveform rhythm in 1 patient and continuous sharp rhythm in 10 patients and slow wave complex or spike and slow wave complex in 4 patient on basic background activity.The epileptogenic focus is located in the left anterior prehippocampus in 8 patients and in the right anterior prehippocampus in 4 patients and in bilateral hippocampal in 4 patients.Conclusions Hippocampal electrode EEG waveform in patients with pharmacoresistant mesial temporal lobe epilepsy was legible.The epileptiform discharges were limited to some electrode contacts.So the epileptogenic zone could be easily determined.If epileptogenic focus could not be determined by scalp EEG or other noninvasive procedures in patients with pharmacoresistant mesial temporal lobe epilepsy,stereotactic implantation of depth electrode into the hippocampus for EEG monitoring might be required.