1. Psychiatric evaluations were made of 56 volunteers for hallucinogen studies. Diagnoses were made on 46% of the group. Forty-one percent were estimated as needing psychiatric treatment. Twenty percent of these had received some form of psychiatric treatment.2. Those subjects with the more effective life adaptations were motivated to volunteer mainly for financial and scientific reasons. Those with diagnosed psychopathology tended to volunteer for other reasons frequently related to their maladaptive patterns.3. The greater the social stimulus to volunteer, the less was the degree of psychopathology present.4. Follow-up interviews frequently revealed psychopathology not ascertained in initial screening interviews. The removal of competitive pressure for selection, the development of positive transferences to the investigators through the hallucinogen experience and the broad time span covered by the follow-ups might explain this.5. Personality patterns covered a wide range with obsessional and schizoid types predominating.6. There was a high degree of agreement among the 3 rating psychiatrists regarding diagnosis and need for treatment. The raters had similar training and experience. They had worked closely together for several years, developing a similar "philosophy" of psychiatry. These factors may have contributed to the high concordance.Abstract Teaser