Objective: To evaluate the suppressive effect of electric cortical stimulatio n upon the seizure onset zone and the non- epileptic cortex covered by subdural electrodes in patients with neocortical epilepsy and mesial temporal lobe epile psy (M- TLE). Methods: Four patients with medically intractable focal epilepsy had implanted subdural electrodes for preoperative evaluation. Cortical function al mapping was performed by intermittently repeating bursts of electric stimulat ion, which consisted of 50 Hz alternating square pulse of 0.3 ms duration, 1- 1 5 mA, within 5 s. The effect of this stimulation on the seizure onset zones and on the non- epileptic areas was evaluated by comparing spike frequency and elec trocorticogram (ECoG) power spectra before and after stimulation. A similar comp arison was performed in stimulation of 0.9 Hz of the seizure onset zones for 15 min. Results: When the seizure onset zone was stimulated with high frequency, sp ike frequency decreased by 24.7% . Logarithmic ECoG power spectra recorded at s timulated electrode significantly decreased in 10- 32 Hz band by high frequency stimulation of the seizure onset zone, and in 14- 32 Hz band by high frequency stimulation of the non- epileptic area. Low frequency stimulation of the seizu re onset zone produced 18.5% spike reduction and slight power decrease in 12- 14 Hz. Conclusions: Both high and low frequency electric cortical stimulation o f the seizure onset zone have a suppressive effect on epileptogenicity. Reductio n of ECoG fast activities after electric cortical stimulation suggests the augme ntation of inhibitory mechanisms in human cortex. Objective: To evaluate the suppressive effect of electric cortical stimulant n upon the seizure onset zone and the non-epileptic cortex covered by subdural electrodes in patients with neocortical epilepsy and mesial temporal lobe epilepsy (M-TLE). Methods: Four patients with medically intractable focal epilepsy had implanted subdural electrodes for preoperative evaluation. Cortical function al mapping was performed by intermittently repeating bursts of electric stimulation, which consisted of 50 Hz alternating square pulse of 0.3 ms duration, 1-1.5 mA, within 5 s. effect of this stimulation on the seizure onset zones and on the non-epileptic areas was as by spike frequency and elec trocorticogram (ECoG) power spectra before and after stimulation. A similar comp arison was performed in stimulation of 0.9 Hz of the seizure onset zones for 15 min. Results: When the seizure onset zone was stimulated with high frequency, spike frequency decreased by 24.7%. Loga rithmic ECoG power spectra recorded at s timulated electrode significantly decreased in 10-32 Hz band by high frequency stimulation of the seizure onset zone, and in 14-32 Hz band by high frequency stimulation of the non-epileptic area. Low frequency stimulation of the seizu re onset zone produced 18.5% spike reduction and slight power decrease in 12-14 Hz. Conclusions: Both high and low frequency electric cortical stimulation of the seizure onset zone have a suppressive effect on epileptogenicity. Reductio n of ECoG fast activities after electric cortical stimulation suggests the augment ntation of inhibitory mechanisms in human cortex.