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To quantify motor disabilities in Parkinson's disease (PD), a compact, portable neurophysiological system based on a personal computer was designed to measure tremor, bradykinesia, and muscle tone. Tremor was detected by solid-state accelerometers and translated into a digital signal. The system displayed the root-mean-square (RMS) displacements and frequency distribution of the tremor in the horizontal and vertical planes, along with a reconstructed graphic image of the displacement. Bradykinesia was measured using a panel that detects release and depression of switches in response to auditory and visual signals; the system calculated subjects' reaction times and movement times in milliseconds. Tone at the elbow was measured by strapping the upper extremity to a lightweight low-friction cradle and then passively moving the cradle with an instrumented handle. Signals representing torque and arm angle were processed by the computer and displayed in real time on the screen with stiffness as a mean slope in Nm/degree. Clinical tests indicate that this system provides a convenient and accurate method for quantifying important aspects of the Parkinsonian syndrome and might be applicable to other movement disorders.