The proposed human experiment will test the validity of a novel paradigm that may provide a method for distinguishing compounds that are likely to have efficacy from those that are not, before they are tested in more costly large-scale clinical trials. Specifically, in a within-subject (cross-over) behavioral pharmacology investigation of 60 smokers, we will test the effects of varenicline versus placebo on smoking behavior in the natural environment following a 3-day period of monitored abstinence and a programmed cigarette lapse. A positive signal for varenicline effects in this study would provide support for the sensitivity of the paradigm. In addition, following completion of all assessments, participants will be followed in a 13-week open-label varenicline smoking cessation trial to determine whether response to varenicline in the laboratory study predicts clinical response in the open-label trial.