For a simple reaction time (RT) task, movement complexity increases RT and also corticospinal excitability (CE), as measured by the motor evoked potential (MEP) elicited by transcranial magnetic stimulation (TMS) of the motor cortex. However, it is unknown if complexity-related increases in CE are mediated at the cortical or spinal level. The purposes of this study were to establish a time-course of motoneuron excitability prior to movement onset and to assess task-dependent effects of complex movements on motoneuron and cortical excitability in a simple RT paradigm. It was hypothesized that motoneuron and cortical excitability would increase prior to movement onset and in response to movement complexity. Participants sat at a KINARM End-Point Lab and completed ballistic elbow extension/flexion movements with their dominant arm to one, two or three targets. TMS and transmastoid stimulation were delivered at 0, 70, 80 or 90% of mean RT for each complexity level. Stimulus intensities were set to elicit motor evoked potentials (MEPs) and cervicomedullary motor evoked potentials (CMEPs) of ~10% of the maximal M-wave in the triceps brachii. Compared to 0% RT, motoneuron excitability (CMEP amplitude) was already greater at 70% RT (23%). CMEP amplitude also increased with movement complexity (p =0.030), which was driven by an increase in motoneuron excitability between the 3-movement and 1-movement conditions (p =0.052). Importantly, when normalized to the CMEP, there was no increase in MEP amplitude. This suggests that complexity-related increases in CE are likely to be mediated more by increased excitability at a motoneuronal than cortical level.