Reversible parkinsonism induced by ataraxics seems to be an effective treatment for schizophrenia, and appears to adhere to Heistad's predictions. If Heistad is correct, the following implications for psychiatry must be faced:1. Submaximal physiological treatment or too rapid return of the internal environment toward normal may fail to achieve good results and may create permanent refractoriness to the treatment procedure.2. Maximal simultaneous change of external and internal environment would be theoretically most efficacious. Waiting for several weeks after hospitalization before starting treatment may decrease a patient's chance for recovery.3. Experiments designed to test the efficacy of any treatment procedure must be done on patients never before hospitalized or treated. Previous hospitalization or treatment would be recognized as exerting a profound influence on any present treatment procedure.