A rapid increase in corticospinal excitability prior to movement is thought to drive agonist muscle activity in simple reaction time tasks using the wrist (MacKinnon & Rothwell, 2000). The purpose of this study was to investigate whether such preparation-related activity occurs before more complex, load-dependent upper limb reaching movements. Ten right-handed subjects (26-45 yrs) performed sessions of elbow flexion in response to a “southwards” (towards the chest) target on a computer screen. The movements were ballistic, planar, centre-out reaching tasks whilst holding a robotic manipulandum and against a constant robot-induced inertial force field (25 N/m). Each session was comprised of 3 blocks (50 trials each; move ratio = 70%, south vs. north (no movement). Single pulse transcranial magnetic stimulation (TMS; 110 % of resting motor threshold) was applied in random order to the contralateral motor cortical biceps representation at 10 time intervals (0-340ms) after the visual GO signal. In a second study (5 subjects), a paired-pulse TMS paradigm was used to investigate short interval intracortical facilitation (SICF; interstimulus interval = 13 ms) and short interval intracortical inhibition (SICI; interstimulus interval = 3 ms) at 3 time points (0,150,200ms) after the visual go signal to the “southwards” target. Peak to peak amplitude of motor evoked potentials (MEPs) from the biceps and triceps muscles were recorded and normalised to MEPs measured at 0ms. Changes in MEP amplitude were tested for statistical significance via repeated measures ANOVA with TMS time interval as the within subject factor. Post hoc students t-tests were corrected for multiple comparisons with an overall significance set at P < 0.05. All data are indicated as mean values ± standard error. Single pulse MEP amplitudes were significantly increased in the biceps 160ms after the visual GO signal in the absence of any increase in background EMG activity (215 +/- 45 %, P<0.05). No significant changes in MEP amplitude occurred in the antagonist triceps (121 +/- 18%, P>0.05). During paired-pulse TMS with a 3 ms interstimulus interval a significant increase in biceps MEP amplitude occurred 150ms after the visual go signal (417 +/- 36 %, P<0.01). With a 13 ms interstimulus interval subjects demonstrated a significant augmentation of biceps MEPs 200ms after the visual go signal (299 +/-39 %, P<0.05). In conclusion, the comparable increase in corticospinal excitability we observed prior to agonist EMG onset in the upper arm compared to more distal musculature during a simple reaction time task suggests that the programs for motor initiation are similar. Furthermore, the pre-movement reduction in SICI and increase in SICF imply that the corticospinal changes are mediated, at least partially, at a cortical level.