THE PRISON IS A MEDIUM SECURITY INSTITUTION FOR YOUNG ADULT MALES. THE STUDY FOCUSED ON TWO QUESTIONS: (1) WHAT IS THE NATURE AND EXTENT OF PERSONALITY CHANGE OF THE INMATE FROM ENTRY TO EXIT; AND (2) IS THE CHANGE A FUNCTION OF THE LENGTH OF STAY IN PRISON. DEPENDENT MEASURES IN THIS STUDY WERE THE MINNESOTA MULTIPHASIC PERSONALITY INVENTORY (MMPI) AND THE CALIFORNIA PSYCHOLOGICAL INVENTORY (CPI), ADMINISTERED TO INMATES UPON ENTRY AND JUST PRIOR TO EXIT FROM THE INSTITUTION. PROFILES OF THESE TWO PSYCHOLOGICAL TESTS WERE EXAMINED BY A PSYCHOLOGIST TO DETERMINE WHETHER OR NOT CHANGE HAD OCCURRED. IN ADDITION, INDIVIDUAL SCORES ON EACH SCALE OF THE TESTS WERE COMPARED FOR CHANGE. ANALYSIS OF THE SCORES SUGGESTED THAT THERE WAS SIGNIFICANT CHANGE AMONG INMATES, AND THAT IT WAS FOR THE BETTER. LOOKING AT THE MMPI, 53 PERCENT OF THE INMATES IMPROVED, 39 PERCENT BECAME WORSE, AND 15 PERCENT DID NOT CHANGE. FOR THE CPI, 43 PERCENT IMPROVED, 32 PERCENT BECAME WORSE, AND 24 PERCENT DID NOT CHANGE. BOTH BLACK AND WHITE INMATES WERE JUDGED AS LESS SOCIALLY DEVIANT AFTER THEIR INCARCERATION. LENGTH OF STAY DID NOT CORRELATE WITH THE CHANGES IN PERSONALITY SCORES. METHODOLOGICAL LIMITATIONS ARE CONSIDERED IN ASSESSING THE FINDINGS. IT IS NOTED THAT THE STUDY DID NOT HAVE A CONTROL GROUP, DID NOT EMPLOY RANDOM SAMPLING, AND DID NOT RULE OUT THE POSSIBILITY OF TESTING EFFECTS. IT IS NOT SUGGESTED THAT THIS STUDY, BY ITSELF, CAN ANSWER THE QUESTION OF WHETHER REHABILITATION WORKS. IT IS SUGGESTED THAT IMPRISONMENT IS BENEFICIAL FOR SOME, BUT HARMFUL FOR OTHERS. SUPPORTING DATA OR REFERENCES ARE NOT INCLUDED. (MLC)