To study the physiological changes in the human brain induced by transcranial magnetic stimulation (TMS), we measured cerebral blood flow (CBF) before and after TMS in 3 healthy volunteers with positron emission tomography (PET). The intersecting point of the figure-of-eight coil was positioned at C3. TMS was delivered 20-32 times at maximum intensity. CBF measured at 50 sec after the termination of TMS showed no significant change in the cerebral cortex corresponding to C3 or any other area. This finding indicates that the hemodynamic change in the brain induced by TMS, if any, lasts only for a short period.