 - All participants in the light or heavy smoker groups must have smoking experience. Those in the heavy smoker group must smoke at least 15 cigarettesday and have smoked for at least the last 2 consecutive years. Those participants in the light smoker group should be situational smokers those who often smoke while consuming alcohol or in another type of situation, but who do not smoke daily. Light smokers should have not smoked more than 1 cigarette in the week before starting the medical screening process and agree to not smoke for the week before the first PET study except for the single cigarette we ask them to smoke. Participants in the heavy smoker group should have experience with abstaining from smoking for at least 48 h. Subjects in the control group must be nonsmokers or have ceased smoking at least two years before the start of the study. 