OBJECTIVE: To determine the effects of the voluntary contraction of muscles and magnetic stimulation intensity on the motor evoked potential (MEP) and the silent period (SP). METHOD: We studied MEPs and SPs in opponens pollicis muscle in 30 healthy adults (male: 16, female: 14) while varying the amount of the voluntary contraction and the stimulation intensity. We analyzed MEPs and SPs in relation to sex, recording site, opposition power and height. RESULTS: 1) During the contraction, the latencies of MEP were significantly shorter than during the relaxation. 2) The amplitudes of MEP reached plateau at 30% of maximal voluntary contraction and increased with increment of stimulation intensity without limitation. The amplitudes of MEP of right hand were bigger than left hand. There were no significant differences according to sex and recording site. 3) The durations of SP were directly proportional to the degrees of voluntary contraction and the stimulation intensity. CONCLUSION: Transcranial magnetic stimulation should be performed under the same voluntary contraction and magnetic stimulation intensity.