The corticomotoneurone pathways were examined in 21 patients with movement disorders, using the technique of percutaneous electrical stimulation of the motor cortex. Conduction in these pathways was assessed by measuring the latency to onset of electromyographic activity in the muscles of the upper limb after cortical stimulation. In all patients [five with primary (idiopathic) torsion dystonia and two with secondary (symptomatic) hemidystonia, seven with Huntington's disease, four with essential tremor, and three with Parkinson's disease] central motor conduction was normal. This and other evidence suggests that the origin of the disorder of movement in these conditions lies in the delivery of abnormal motor commands to a normal corticomotoneuronal system.