1. Changes in excitability of the motor cortex induced by a transcranial magnetic stimulation (TMS) were examined by simultaneous recording of the evoked corticospinal volley and the compound surface electromyographic (EMG) response in the biceps brachii following paired-pulse TMS in five conscious subjects. The effects of a varying interstimulus interval (ISI) and a conditioning stimulus intensity were also investigated. 2. A submotor threshold conditioning stimulus inhibited the test responses at ISIs of 2-5 ms. A supramotor threshold conditioning stimulus inhibited the test responses at ISIs of 100-200 ms. Both of these inhibitions were prominent in late I waves. 3. There was a facilitation of the test responses at an ISI of 25 ms that was prominent in late I waves. The facilitation evoked by the supramotor threshold conditioning stimulus was more prominent than that evoked by the submotor threshold conditioning stimulus. 4. It is concluded that single TMS induced the triphasic changes of the motor cortex excitability in conscious humans that resulted in changes in EMG responses following paired TMS.