Short- and long-latency responses (HR and LLR) from thenar muscles were studied in patients with Friedreich's ataxia and pure cerebellar ataxia with later onset by applying electrical stimuli on the median nerve at the wrist. HR and LLR were examined during two different voluntary activities of the opponens pollicis muscle: isometric ("hold") and isotonic ballistic ("move") conditions. A preliminary conventional study of motor and sensory conduction of the median nerve was also carried out. Patients with Friedreich's ataxia had reduced or absent HR and LLR. Furthermore, those who preserved both responses had prolonged HR-LLR interpeak latency. All patients with Friedreich's ataxia also showed peripheral nerve conduction abnormalities, mainly in sensory fibers. These data can be accounted for by the widespread degeneration of many neural structures in this disorder. No abnormalities in HR were observed in pure cerebellar ataxia with later onset, whereas LLR was grossly enlarged in most patients, notably during "move" condition. Since cerebellar structures (especially the cerebellar cortex) are the only ones involved in this disorder, the cerebellum may play a role in modulating LLR. In particular, this effect could be more evident in isotonic ballistic movements.