Seven dystonic and eight athetotic patients with cerebral palsy were clinically diagnosed following the classification of motor symptoms of the American Academy for Cerebral Palsy, and examined with surface electromyography while at rest in the supine position, under mental stress, making voluntary contractions and responding to passive stretch. Involuntary movements of both dystonic and tension athetotic cerebral palsy were characterized by nonreciprocal involuntary muscle activity in agonists and antagonists. There was more tonic involuntary muscle activity in dystonic than in tension athetotic cerebral palsy patients. During involuntary movements in three dystonic patients, muscle action potentials recorded simultaneously in different muscles had a uniform pattern of duration and amplitude. Electromyographical findings under mental stress were almost the same as those at rest in both dystonic and tension athetotic forms, except that the amplitude and duration of muscle activity was larger under mental stress than at rest. When both dystonic and tension athetotic patients were asked to make voluntary contractions, they were unable to do so smoothly, because involuntary movements were induced in agonists and antagonists. Especially in dystonic patients, voluntary efforts induced involuntary movements not only in agonists and antagonists but also in other muscles not concerned with voluntary contraction. In all dystonic patients, the responses to passive stretch were characterized by rigidity or rigidospasticity. In five tension athetotic patients, the stretch reflex was characterized by spasticity, but in three patients the stretch reflex was not seen at all. Therefore, it is assumed that dystonic movements arise from rigidity or rigidospastic hypertonus. In three dystonic patients, the stretch reflex brought about action potentials not only in antagonists of the stretched muscle, but also in other muscles not concerned with passive stretch. These synchronized action potentials were neither of involuntary muscle activity induced by passive stretch nor of paradoxical contraction of Westphal. It is presumed that such action potentials are induced by suprasegmental central mechanisms closely related to the stretch reflex.