2. History of Drug Abuse: Volunteers reporting current or having a significant history of illicit drug abuse (single illicit substance use of more than 10 times in a lifetime for any given substance, except marijuana) will be excluded from the study. Volunteers reporting current use of nicotine will be excluded because nAChRs are usually elevated in smokers, and the purpose of this protocol is to determine the feasibility of measuring nAChRs in vivo. Including smokers at this time may provide results that are more variable and which may overestimate our ability to measure these receptors in nonsmokers. Subjects may use moderate amounts of alcohol and caffeine and smoke an occasional marijuana cigarette. Moderate alcohol use will be defined as less than ten drinks of liquor (1.5 oz) or the equivalent beer (12 oz) or wine (5 oz) per week. Moderate caffeine use will be defined as less than 500 mg of caffeine per day, where 100 mg is equivalent to 1 serving of coffee (5 oz serving), 2.5 servings (12 oz serving) of caffeinated soft drinks, or 2.5 servings of tea (5 oz serving). Occasional marijuana use will be defined as less than or equal to two marijuana cigarettes/month.