Twelve subjects with progressive myoclonus epilepsy (PME) were studied with transcranial magnetic stimulation (TMS), using single and paired magnetic stimuli at different interstimulus intervals (ISIs), and polygraphic recording. Motor threshold (T) and silent period (SP) were normal. Paired TMS showed a loss of inhibition at 100-150 ms ISI and a marked facilitation at 50 ms ISI of conditioned motor evoked potential (MEP). Polygraphic analysis showed 20 Hz oscillatory activity over the sensorimotor area coupled to contralateral myoclonic jerks. These findings suggest a condition of increased supraspinal excitability and support the evidence of a cortical rhythm in the range of 20 Hz. No direct evidence exists that these findings are mediated by the same intracortical pathway. Furthermore, the normal SP and T suggest that the abnormal excitability is not a constant feature but is evident during rhythmic events.