The study proposed is a 22 to 23 day, double-blind, placebo-controlled crossover study in 60 male and female moderate to heavy alcohol users who are Asp40 positive and negative. Subjects will undergo a 6 to 7-day cycle taking the first study drug (naltrexone or placebo) with an alcohol self-administration visit in the laboratory bar on the 6th or 7th day of drug. The first alcohol self-administration visit is followed by an 8 to 9 day washout period. Subjects then cross over to repeat the cycle while taking the other drug (placebo or naltrexone). On the final day (either day 8 or 9) of the first study drug cycle, the subject will undergo functional imaging (fMRI) at the WBIC. There will be at least one day in between the 'lab bar' and the fMRI scan. The first fMRI visit is followed by a 5-6 day washout period. Subjects then cross over to repeat the cycle while taking the other drug (placebo or naltrexone). Subjects will keep daily diaries of alcohol and medication use throughout the study.