An electromyographic technique was used to record the tonic stretch reflex (TSR) responses to passive sinusoidal stretching of biceps brachii muscle in athetotic patients. The TSR transmission characteristics measured while the patient was in a relaxed state were compared with those measured while the patient was voluntarily active. The resting TSR and action TSR were found to differ markedly. Sensitivity, pattern, duration, and timing of the TSR responses were all changed during voluntary activity. The results suggest that the TSR in athetosis is functionally reorganized by supraspinal influences during voluntary activity. It is proposed that athetoid movements and action tremor may be a manifestation of abnormal action TSR caused by faulty supra-spinal control during activity.