In patients with dystonia, abnormal movements are commonly triggered or made worse by voluntary action. By means of transcranial magnetic stimulation (TMS), we investigated changes in motor cortex excitability before the execution of wrist voluntary movements in patients with upper limb dystonia and normal control subjects. Magnetic stimulation was delivered by two Magstim 200 stimulators connected through a Bistim module to a figure-of-eight coil placed over the motor area of the forearm extensor muscles. A subthreshold (80% of the rest motor threshold) conditioning stimulus was delivered 3 ms before the suprathreshold (120% of the rest motor threshold) test stimulus and the degree of inhibition of the conditioned motor evoked potentials (MEPs) was taken as an indicator of intracortical inhibition. MEPs were recorded over the forearm extensor muscles of the right arm. To study MEP amplitudes and intracortical inhibition before the onset of wrist extension in the pre-movement condition, TMS pulses were delivered from 0 ms to 100 ms after the go-signal. Besides the pre-movement condition, intracortical inhibition and the unconditioned MEP size were also investigated at rest and during tonic wrist extension. In healthy subjects studied before the wrist movement, the unconditioned MEP amplitude increased progressively and intracortical inhibition decreased significantly. Before movement in dystonic patients, the unconditioned MEP amplitude remained significantly unchanged from resting values and intracortical inhibition decreased less than it did in healthy subjects. In both groups studied during contraction, the unconditioned MEP amplitude increased and intracortical inhibition decreased from values at rest. In conclusion, these findings from reaction time tasks in patients with primary dystonia provide evidence of abnormal pre-movement motor cortex excitability. This abnormality is due to an altered release or running of motor programmes.