Careful evaluation of the study hypotheses will provide important information on the efficacy and mechanism of the effects of targeted naltrexone in problem drinkers. This study will allow us to model effects across multiple levels of analysis in an effort to understand the psychopharmacological mechanisms underlying the therapeutic effects of naltrexone in problem drinkers and to apply novel genetic findings to understanding the pharmacogenetic mechanisms underlying the therapeutic effects of naltrexone in problem drinkers.