Tobacco Smoking is the most prevalent addiction in society today causing directly major health hazards and sharing morbidity with other psychiatric disorders. Nicotine binds to acetylcholine receptors and thus elevates DA release and inhibits DA transport. There are few studies using advanced brain imaging techniques to investigate how nicotine releases dopamine in humans. These studies utilized dopamine displacement paradigms with [11C] Raclopride binding to D2 receptor in Positron Emission Tomography (PET). There is evidence that smokers (particularly those who enjoyed smoking) showed decreased [11C] Raclopride binding in the caudate/nucleus accumbens and putamen after smoking cigarettes. These results also indicated that the effects of nicotine on dopaminergic neurotransmission are mediated by pleasure and craving. We propose to investigate the effects of smoking a cigarette ad lib on dopamine release by using dopamine competition paradigm with [I123] IBZM in SPECT. Secondly, we will test the hypothesis that dopamine deficiency is a major vulnerability factor for smoking. This may provide further evidence that dopamine deficiency in some smokers pre-disposes them to enjoy and desire smoking cigarettes more than others.