In a simple reaction time (RT) paradigm, magnetic stimulation of different intensities was delivered over different scalp positions and at variable delays before (negative) or after (positive) the go-signal. Magnetic stimulation shortened RT to different go-signals (auditory, visual and somatosensory stimuli) by approximately 30 ms when delivered over the motor cortex contralateral to the responding arm at intensities below motor threshold. This effect was maximal at a delay of approximately +10 ms. A similar effect was found with suprathreshold stimulation to the ipsilateral motor cortex. Magnetic stimulation over other scalp areas did not affect RT regardless of the delay. No differences were found between the effects on elbow flexion and thumb abduction. The shortening of RT was not associated with changes in the timing development of premovement excitability increase in the motor cortex. We conclude that magnetic stimulation shortens RT by inducing an earlier initiation of this excitability increase.