This study will demonstrate the behavioral responses to varenicline, helping to better understand its mechanisms. Hypotheses for the study are to observe decreases in smoking topography and nicotine cigarette choice on varenicline, relative to placebo; and decreases on day 21 relative to day 7 during varenicline treatment. We also propose to examine if extended duration of treatment has clinical significance in decreasing smoking behaviors, thus increasing the efficacy of varenicline.