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The aim of this study is to investigate the efficacy of 1-Hz repetitive transcranial magnetic stimulation (rTMS) over the primary motor cortex (M1) on acute pain induced by intradermal capsaicin injection and to elucidate its mechanisms by single-photon emission computed tomography (SPECT). We compared time courses of a subjective scale of pain induced by intradermal capsaicin injection in seven normal subjects under three different conditions: rTMS over M1, sham stimulation, and control condition (natural course of acute pain without any stimulation). In ten normal subjects, using SPECT, we also studied differences in regional cerebral blood flow (rCBF) after capsaicin injection between two conditions: rTMS over M1 and the control condition. rTMS over M1 induced earlier recovery from acute pain compared with the sham or control conditions. Under rTMS over the right M1 condition compared with the control condition, the SPECT study demonstrated a significant relative rCBF decrease in the right medial prefrontal cortex (MPFC) corresponding to Brodmann area (BA) 9, and a significant increase in the caudal part of the right anterior cingulate cortex (ACC) corresponding to BA24 and the left premotor area (BA6). A region-of-interest analysis showed significant correlation between pain reduction and rCBF changes in both BA9 and BA24. We conclude that rTMS over M1 should have beneficial effects on acute pain, and its effects must be caused by functional changes of MPFC and caudal ACC.