The inhibitory control of prepotent action is vital for appropriate behaviour. An example of the importance of such control can be seen in the inhibition of aggressive behavior, deficits in which may have broader consequences for society. Many studies have related lesions or the under-development of the prefrontal cortex to inefficiency of inhibitory control. Here we used transcranial magnetic stimulation and a stop-signal task, which occasionally requires the inhibition of a prepotent motor response, to investigate the role of pre-supplementary motor area (Pre-SMA) in inhibitory control. While no effects were seen on the ability to generate responses, TMS delivered over the Pre-SMA disrupted the ability to respond to a stop signal. These results are the first to establish a casual link between Pre-SMA and inhibitory control in normal subjects. The understanding of the underlying mechanisms of inhibitory control may lead to clearer understanding of the neural basis of inappropriate behaviour.