48 patients with medically intractable epilepsy were submitted to a total of 60 series of transcranial magnetic stimulation. Effects of transcranial magnetic stimulation were monitored via subdural electrodes. The aim of the study was to investigate the use and reliability of the method for the localization and delineation of the epileptic focus in the process of presurgical evaluation of epilepsy. The following effects of transcranial magnetic stimulation could be observed: (i) Induction of complex partial seizures or auras during (n = 3) or within 5 min after (n = 3) serial stimulation. (ii) Necessity of the presence of additional provocative circumstances for the induction of seizures. (iii) Induction of short-lived epileptiform afterdischarge potentials selectively in the epileptogenic area (or areas) (n = 22). (iv) Enhancement as well as suppression of epileptiform potentials in patients with continuously spiking epileptic foci (n = 7). (v) Temporary interruption of epileptiform paroxysms for 1-3 sec (n = 9). (vi) Persistent suppression of spontaneous spikes (n = 1). (vii) Induction of focal theta or delta activity (n = 26). (viii) No clear-cut response of the epileptic focus (n = 18). (ix) Action of anticonvulsant medication against epileptiform afterdischarge or seizure induction. In conclusion, transcranial magnetic stimulation provides complementary information about the location of the epileptic focus in the process of presurgical evaluation. However, its practical use is limited mainly by its low capability to induce seizures.