The purpose of this study was to investigate the time-course of corticospinal excitability during reaction time (RT), and compare the excitability when a precue provided information regarding both the direction and extent of the upcoming movement ('Full' condition), specified the direction of the upcoming movement only ('Direction' condition), or provided no information at all ('None' condition). Ten healthy, right-handed subjects performed a four-choice RT task that involved flexion and extension of dominant wrist. Transcranial magnetic stimulation (TMS) was presented at random intervals over a period of 120 ms prior to the subject's average non-stimulated voluntary electromyography (EMG) activity onset. We found that there was a significant relationship between motor-evoked potential (MEP) amplitude and TMS onset when both the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) acted as an agonist. This relationship could be explained using the sigmoidal Boltzmann equation. The slope for the relationship did not differ between the Full and Direction conditions, suggesting that corticospinal excitability is not altered in the specification of movement extent. Both these conditions differed significantly from the None condition. The modulation of corticospinal excitability appeared greater in the FCR than in the ECR. There was a significant delay in RT the closer in time TMS was presented with respect to EMG onset. During extension, there was no difference in slope between the three conditions, whereas during flexion the slope was greater in the None condition than in the Direction condition, which was in turn greater than in the Full condition. This was mirrored in the relationship between agonist MEP amplitude and TMS onset for both muscles. It is possible that the gain of the corticospinal tract is increased in the conditions in which less information is provided in the precue to partly compensate for the increase in RT, which comes as a result of the additional processing required in those conditions.