Ia reciprocal inhibition and presynaptic inhibition are related to motor control. The effect of corticospinal excitability to on the spinal inhibitory circuit is studied by repetitive transcranial magnetic stimulation (rTMS). However, a relationship between corticospinal excitability and spinal inhibitory circuit has not been considered sufficiently. In recent years, quadripulse transcranial magnetic stimulation (QPS) has attracteds attention as a new technique to induce neural plasticity. QPS induce to increases a corticospinal excitability more effectively than does rTMS. The aim of this study was to clarify the relationship between corticospinal excitability and inhibition of the H- reflex. In Experiment 1, control H-reflex of the flexor carpi radialis (FCR) was induced by stimulation of the median nerve, and the test stimulus intensity was modulated so that the H-reflex was 20% of M max. To assess the Ia reciprocal inhibition and presynaptic inhibition, we applied conditioning stimulation to the radial nerve. The intensity of conditioning stimulation was set to 90% intensity of the M-wave threshold. The interstimulus interval (ISI) was set at 0-5,10,12,14,16,18,20,25,or 30 ms. The conditioned H-reflex was normalized to control H-reflex (% control amplitude). ISI-Ia was set to the minimum % control amplitude from ISI of 0 to 5 ms, in the same way ISI-presynapse was determined from ISI of 10-30 ms. In Experiment 2, the motor evoked potential (MEP) amplitude and H-reflex of the FCR were measured. To measure the conditioned H-reflex, we adopted the ISI-Ia and ISI-presynapse. QPS was performed for 30 minutes. QPS intensity was set to 90% of the motor threshold. We measured the H-reflex and MEP amplitudes twice before QPS (pre QPS-1, pre QPS-2) and every 20 minutes after QPS (post QPS-0, post QPS-20, post QPS-40, post QPS-60). The ratio of % control amplitude to pre QPS-1 was calculated (% pre1 amplitude). A one-way repeated measures analysis with time period as a factor was used to determine the effect of QPS on MEP amplitude and % pre1 amplitude (p<0.05). Dunnett multiple comparisons were performed as post hoc tests (p<0.05). In Experiment 1, The average of ISI-Ia was 4.5 ms and the average of ISI-presynapse was 20 ms. In Experiment 2, the MEP amplitude in post QPS-0 increased significantly compared with that in pre QPS-1 (p=0.01). The % pre1 amplitude of ISI-presynapse in post QPS-0 decreased significantly compared with that in pre QPS-2 (p=0.021). Although the differences were not statistically significant, the MEP amplitude in post QPS 20 increased and the % pre1 amplitude of ISI-presynapse in post QPS 20 decreased. These experimental results suggested that increased corticospinal excitability and increased presynaptic inhibition at the spinal level occurred in parallel.