In 10 patients with Parkinsonism, treatment with L-dopa was associated with a rise in the skin conductance level and its fluctuations, an increased electromyographic activity in the frontalis muscle, and with faster reaction times. The change of skin conductance in response to a loud noise also increased and its habituation to the repetition of the stimulus was delayed. One patient developed a schizophreniform psychosis and showed high arousal and absent habituation. It is possible that dopamine is the transmitter associated with the arousal reaction and that it plays a part in the pathogenesis of schizophrenia.