In 9 normal volunteers, we studied the safety of rapid-rate transcranial magnetic stimulation (rTMS) applied to different scalp positions at various frequencies and intensities. Pure tone threshold audiometry showed temporary threshold shifts in 3 subjects. In the subject stimulated at the highest intensity, rTMS induced a focal, secondarily generalized seizure despite the absence of definite risk factors for seizures. Rapid-rate TMS did not result in any important changes in the neurological examination findings, cognitive performance, electroencephalogram, electrocardiogram, and hormone levels (prolactin, adrenocorticotropic hormone, thyroid-stimulating hormone, luteinizing hormone, and follicle-stimulating hormone). In 10 additional subjects, the electromyographic activity in several contralateral muscles showed that trains of rTMS applied to the motor cortex induced a spread of cortical excitability. The spread of excitability depended on the intensity and frequency of the stimuli and probably constituted an early epileptogenic effect of rTMS. Guidelines for preventing the undesirable side effects of rTMS are offered.