Seven patients with cortical action myoclonus were studied. Six of these also had cortical reflex myoclonus. Surface and needle recordings were made from intrinsic hand muscles. Reflex and action jerks often consisted of a series of EMG bursts, some of which were polyphasic. The interval between repetitive EMG bursts was most commonly approximately 20 ms. The first EMG burst following median nerve stimulation varied in latency by a few milliseconds. Needle recordings of this reflex response showed more than one peak in half of the post-stimulus time histograms (PSTH) collected. Multiple peaks were narrow and separated by as little as 4 ms. Single trials of the EEG activity over the sensorimotor cortex were recorded during stimulation or voluntary movement of the contralateral limb in five patients. Activity typically consisted of a rhythmic series of giant positive spike-slow negative wave complexes. Intervals between spikes tended to cluster at approximately 20 ms. Frequency histograms of spike to EMG burst intervals confirmed that spikes preceded muscle discharges but they showed more than one peak. These peaks were narrow and separated by as little as 3 ms. Thus, in patients with cortical myoclonus rhythmic muscle responses are driven by the sensorimotor cortex, which has a tendency to oscillatory activity. Two types of cortical rhythmicity are present. The first determines the frequency of repetitive EEG spikes and EMG bursts. The second is of higher frequency and underlies the multiple peaks separated by short intervals in PSTHs and histograms of spike to EMG burst intervals. Both phenomena may have their correlate in normal functioning.