 - History of Drug Abuse: Volunteers reporting current or having a significant history of illicit drug abuse single illicit substance use of more than 30 times in a lifetime for any given substance, except marijuana will be excluded from the study. However, if the use was greater than 10 times in a lifetime, it must have occurred more than 10 years before enrollment in this protocol. Subjects may use moderate amounts of alcohol and caffeine and smoke an occasional marijuana cigarette. Moderate alcohol use will be defined as less than fourteen 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 cigarettesmonth. 