Patients with cerebellar degeneration and patients with Parkinson's disease (PD) have prolonged reaction times (RT). In PD, the motor cortex excitability build-up preceding a movement is abnormal. This study tested the hypothesis that patients with cerebellar degeneration also have deficits in the pre-movement excitability build-up of the motor cortex. In addition, we evaluated if transcranial magnetic stimulation (TMS) could improve RT by pre-activation of the motor cortex. Ten patients with cerebellar degeneration and 10 age-matched normals performed a reaction time task making wrist flexions as a response to a visual go-signal. In order to test the pre-movement motor cortex excitability, sub-threshold TMS was delivered at various intervals (0-250 ms) after the go-signal. The number of motor evoked potentials was determined in relation to the EMG onset, thus indicating excitability at different intervals prior to the EMG activity. In 50% of the trials, an additional TMS was given simultaneously with the go-signal to test if this pre-activation of the motor cortex changes the excitability build-up. The possibility of improving RT was tested by delivering single subthreshold TMS in different intervals close to the go-signal. The RT was prolonged in cerebellar patients compared to the control group (247 vs 200 ms, p = 0.0015). The pre-movement excitability started 100 ms earlier in the patient group than in the control group and showed a slow increase. With the additional TMS, the excitability build-up in the patient group shortened and became similar to the control group. The RT improved by up to 40 ms (p < 0.01) when TMS was given close to the go-signal. We postulate that the prolongation of RT in cerebellar degeneration is mainly due to an abnormal excitability build-up of the motor cortex. Pre-activation of the motor cortex by TMS is able to improve the excitability build-up and the RT in cerebellar degeneration.