Ten hypokinetic parkinsonian patients and ten age- and sex-matched controls were tested, using reaction times with and without a warning signal. The time difference between each was used as a measure of phasic arousal. Both groups' reaction times improved with a warning signal, but there were no differences in reaction-time reduction between the parkinsonian and control groups. These results suggest either that hypokinesia in parkinsonism is not caused by an arousal defect or it is caused by a defect in the “autoevoked” arousal system. © 1976, American Medical Association. All rights reserved.