The aim of this study was to evaluate corticospinal excitability of both hemispheres during the reaction time (RT) using transcranial magnetic stimulation (TMS). Nine right-handed subjects performed right and left thumb extensions in simple (SRT), choice (CRT) and go/no-go auditory RT paradigms. TMS, inducing motor-evoked potentials (MEPs) simultaneously in the extensor pollicis brevis muscles bilaterally, was applied at different latencies from the tone. For all paradigms, MEP amplitudes on the side of movement increased progressively in the 80-120 ms before EMG onset, while the resting side showed inhibition. The inhibition was significantly more pronounced for right than for left thumb movements. For the left SRT, significant facilitation occurred on the right after EMG onset. Initial bilateral facilitation occurred in SRT trials with slow RT. After no-go tones, bilateral inhibition occurred at a time corresponding to the mean RT to go tones. The timing of the corticospinal rise in excitability on the side of movement was independent of task difficulty and RT. This suggests that corticospinal activation is, to some extent, in series and not in parallel with stimulus processing and response selection. Corticospinal inhibition on the side not to be moved implies that suppression of movement is an active process. This inhibition is more efficient for right- than for left-side movements in right-handed subjects, possibly because of left hemispheric dominance for movement.