The study uses a mixed factorial design with one between subject factor (OPRM1 genotype_: Asn40/Asn40 vs. Asn40/Asp40 or Asp40/Asp40) and one within-subject factor (IV nicotine vs. IV saline) to examine genotype by nicotine interactions on MOR binding potential (BP_ND ) assessed via PET imaging with [11 C]carfentanil. Twenty-four smokers (12 male, 12 female; 12 from each genotype group) will participate in two 90 minute PET sessions following overnight (14-hours) abstinence from nicotine. Genotype groups will be matched for age and sex . One week prior to the first PET session, there will be an adaptation session during which participants will receive IV saline followed 30 minutes later by IV nicotine (1 mg/70 kg) to ensure that they tolerate the procedure. In the PET sessions, participants will receive either IV nicotine (1 mg/70 kg) or saline (within-subject, double blind, counterbalanced). The primary outcomes will be BP_ND in ventral striatum and anterior cingulate cortex (ACC). Normally menstruating women will be scheduled for their sessions during the early follicular phase. Sessions will be separated by 1 month for all participants to reduce variability in MOR binding due to hormonal changes during females menstrual cycles. Participants will complete subjective measures of nicotine reward and craving at each session.