To establish whether transcranial magnetic stimulation is able to activate the primary epileptic focus preferentially, 13 patients who had medically intractable complex partial seizures were examined prior to surgical therapy. Single or a series of magnetic stimuli were applied to various regions of the skull. The effects of transcranial magnetic stimulation were monitored via subdurally implanted electrodes. In the process of presurgical evaluation, the dosage of anticonvulsant medication had been reduced in all patients but one. Transcranial magnetic stimulation was able to activate the epileptic focus (or foci) in 12 of the 13 patients. Distinct patterns of focal activation were observed in 3 patients who had several foci. No epileptiform potentials were induced outside epileptic foci, which had been identified by corticographic recordings. In one patient a complex partial seizure that was induced was identical to her habitual seizures. In another patient, a complete transition from a nonactive theta focus to a self-sustained epileptic focus occurred. A facilitation of epileptiform afterdischarge was seen with sequential stimulation. No adverse effects were either reported by the patients or observed by the investigators. In summary transcranial magnetic stimulation is able to activate the epileptic focus (or foci) and consequently may be an additional tool for the localization of epileptic foci in presurgical evaluation.