RANDOM SAMPLES WERE SELECTED FROM ARMY-ENLISTED MALES WHO RETURNED FROM VIETNAM IN SEPTEMBER 1971 AND FROM A LIST OF MEN RETURNING THAT MONTH FOR WHOM URINE ANALYSIS INDICATED HEROIN USE. INITIAL INTERVIEWS WERE CONDUCTED WITH 943 MEN. FOLLOWUP INTERVIEWS WERE CONDUCTED WITH APPROXIMATELY TWO-THIRDS OF THE SOLDIERS AFTER THEY HAD BEEN HOME FOR 3 YEARS. INTERVIEWS WERE ALSO CONDUCTED WITH A MATCHED SAMPLE OF NONVETERANS. URINE SAMPLES WERE ANALYZED TO VERIFY SELF-REPORTS ABOUT DRUG BEHAVIOR. PREDISPOSITION FACTORS CONSIDERED INCLUDE DEMOGRAPHIC TRAITS SUCH AS RACE, SIZE OF CITY OF REARING, AND AGE AFFECTING DEGREE OF EXPOSURE TO DRUG OPPORTUNITIES PRIOR TO MILITARY SERVICE AND PRESERVICE DEVIANCE. DESPITE UNEXPECTED DIFFERENCES IN DRUG USE BASELINE MEASURES FOR EXPERIMENTAL SUBJECTS AND MATCHED CONTROLS, THE ANALYSIS SUGGESTS THAT THE VIETNAM SETTING PROMOTED INITIATION OF THE USE OF NARCOTICS AND MARIHUANA BUT NOT OF BARBITURATES AND AMPHETAMINES. THERE WAS A TEMPORARY REDUCTION IN INITIATIONS OF MARIHUANA AND NARCOTICS USE AMONG VETERANS IMMEDIATELY UPON RETURN FROM VIETNAM, FOLLOWED BY A LATER REBOUND. THIS FINDING SUGGESTS THAT THE VIETNAM SETTING FACILITATED INTRODUCTIONS TO THESE DRUGS SO THAT ALMOST ALL THOSE PREDISPOSED TO USE DURING THAT TIME PERIOD WERE SUFFICIENTLY EXPOSED, LEAVING A TEMPORARILY 'IMMUNE' POPULATION AMONG RETURNING SOLDIERS. FURTHER SUPPORT FOR THE CHANGE IN SETTING EFFECT IS FOUND IN THE FACT THAT THE VIETNAM EXPOSURE HAD BOTH A PERMANENT EFFECT ON NUMBER OF USERS AND A POSTSETTING 'IMMUNITY' EFFECT FOR TWO CLASSES OF DRUGS KNOWN TO BE AVAILABLE IN VIETNAM, BUT NOT FOR OTHER DRUGS. RATHER THAN EQUALIZING THE DRUG EXPERIENCES OF MEN FROM DIFFERENT SOCIAL AND BEHAVIORAL BACKGROUNDS, EASY ACCESS TO NARCOTICS IN VIETNAM SEEMED TO INCREASE THEIR PREEXISTING DIFFERENCES. RELATIVE TO NARCOTIC AND MARIHUANA USE, A SETTING WIH GREATER OPPORTUNITIES TO EXPRESS DEVIANT BEHAVIOR INCREASED THE IMPACT OF PRIOR PREDISPOSITIONS TO DEVIANCE. SUPPORTING DATA AND A LIST OF REFERENCES ARE INCLUDED. (LKM)