Four patients with cortical myoclonus were studied. All had reflex muscle jerking and grossly enlarged somatosensory evoked responses (SEPs) following electrical stimulation of the digital nerves. In addition, three of the patients had spontaneous or action-induced myoclonus. Back-averaging the EEG from these spontaneous muscle jerks showed a large positive wave over the contralateral somatomotor cortex which preceded the jerk by about 20 ms. Administration of lisuride (0.1 mg iv) reduced the severity of the reflex and spontaneous myoclonus, but had no effect on, or increased the size of the SEP. Two of the patients also received 1 mg clonazepam iv. As with lisuride, the severity of myoclonic jerking was reduced although the size of the SEP was increased. It is concluded that the usual association between giant SEPs and reflex muscle jerking can be abolished by acute administration of lisuride and clonazepam in patients with cortical myoclonus.