Reflex activity in the biceps and triceps muscles evoked by applied torque perturbations was studied in patients with Parkinson's disease. The perturbations consisted of single pulses or of pseudo-random sequences of pulses of torque. The patients were treated with levodopa and some exhibited marked fluctuations in their clinical disabilities ("on-off" effect). The study was undertaken to see if reflex activity changed in parallel with the fluctuations of their clinical symptoms. It was found that the reflex activity in these patients could be classified into two types, a Type I response differing little from normal and a Type II response exhibiting marked high-frequency (8-14 Hz) oscillations in EMG activity. Both Type I and Type II responses were virtually the same in the "on" as in the "off" state.