The nicotine metabolite ratio (NMR), a stable marker of nicotine clearance rate, is a robust predictor of smoking relapse. Individuals who are fast nicotine metabolizers have higher rates of relapse, compared to slow metabolizers, on nicotine replacement or placebo treatment. Nicotine exerts its reinforcing properties, in part, by binding to α4β2* nicotinic acetylcholine receptors (nAChRs) in the brain. The α4β2* nAChRs are abundant and have high affinity for nicotine relative to other nAChR subtypes. The goal of this project is to identify abstinence-induced changes in neuronal nicotinic receptor availability that may underlie risk for smoking relapse.