 Smokers will be randomized into one of the following 3 groups: a complete tobacco abstinence for 8 days; b denicotinized research cigarettes for 8 days, or c control group smoking nicotinized research cigarettes during the entire study 16 days. Because there are few studies comparing functional brain activity in smokers and nonsmokers, a group of nonsmokers will serve as a control group for the brain imaging studies. 