STUDY SUBJECTS WERE 37 INMATES WHO HAD VOLUNTEERED FOR A DRUG-TESTING PROGRAM AND 33 INMATES WHO HAD NOT VOLUNTEERED. THE SUBJECTS WERE INTERVIEWED BY A PSYCHOLOGIST TO OBTAIN DEMOGRAPHIC AND MOTIVATIONAL DATA. THE INMATES COMPLETED A STANDARD PSYCHOPATHOLOGY INVENTORY (THE MINNESOTA MULTIPHASIC PERSONALITY INVENTORY), THE SIMILARITIES AND PICTURE ARRANGEMENT SUBTESTS OF THE WECHSLER ADULT INTELLIGENCE SCALE, A TEST OF NORMAL PERSONALITY VARIABLES (THE EDWARDS PERSONALITY SCALE),AND CHRISTIES' MACHIAVELLIANISM SCALE (A MEASURE OF MANIPULATIVE TENDENCIES). THE FINDINGS INDICATE THAT INMATES ARE GENERALLY FREE ENOUGH FROM PSYCHOPATHOLOGY TO MAKE RATIONAL CHOICES ABOUT PARTICIPATING IN MEDICAL RESEARCH, AS LONG AS PRELIMINARY SCREENING IS CARRIED OUT TO ELIMINATE INDIVIDUALS WITH SEVERELY DISORDERED THINKING. VERBAL COMMUNICATION SKILLS OF INMATES APPEAR TO BE WELL ENOUGH DEVELOPED TO ALLOW THEM TO GIVE AN INFORMED CONSENT. ALTHOUGH THERE ARE PERSONALITY DIFFERENCES BETWEEN VOLUNTEERS AND NONVOLUNTEERS, THESE APPEAR TO RELATE PRIMARILY TO THE FACT THAT, WITHOUT SCREENING, IMPULSIVE, MANIPULATIVE INDIVIDUALS WITH DISORDERED THOUGHT PROCESSES ARE OVERREPRESENTED AMONG VOLUNTEERS FOR MEDICAL RESEARCH. SUPPORTING DATA AND A LIST OF REFERENCES ARE INCLUDED. (AUTHOR ABSTRACT MODIFIED--LKM)