The primary purpose of the study is to increase our knowledge of receptor function in the brains of people who are heavy drinkers and taking naltrexone (NTX), a medication that has been approved for the treatment of alcohol dependence. Receptors are special molecules in the brain to which other molecules (neurotransmitters) attach during the normal every-day workings of the brain. Drugs can bind to those receptor molecules as well. Recent evidence suggests that kappa opioid receptors (KOR's) may play an important role in alcohol drinking behavior. This study will try to determine if naltrexone's ability to attach to these receptors is related to its effectiveness. We will use PET (positron emission tomography) for this study. PET is a type of imaging device found in nuclear medicine. It is used for tracking the presence of injected radioactive materials in the body.