In this study we investigated the physiologic mechanisms in primary brachial dystonia by analyzing the cutaneous EMG silent period during isometric contraction of the opponens pollicis muscle. Results from the affected and unaffected arms of 11 patients with dystonia were compared to 7 patients with Parkinson's disease and 16 age-matched normal individuals (controls). The silent period onset latency, degree of EMG suppression during the silent period, and EMG rebound at the end of the silent period did not differ significantly between patients with dystonia and any other group. The duration of the silent period (the S-X interval), however, was significantly prolonged in dystonia (p<0.005) and in Parkinson's disease (p<0.001) in both affected and unaffected arms compared with controls. These findings suggest that mechanisms responsible for the initiation of the cutaneously induced silent period and the subsequent suppression depth of EMG activity are not affected in brachial dystonia, but the abnormally prolonged S-X intervals may reflect dysfunctional basal ganglia timing influences over spinal circuitry common to both dystonia and Parkinson's disease.