 The study is divided into 5 separate arms with a maximum of two arms being conducted at one time. The experimental arms explore the effect of intravenous nicotine delivery with respect to the following parameters: dosage, interval of delivery, rate of delivery and emotional processing. Specifically the arms are as follows: Experiment 1: 3 fMRI scan sessions each administering either 0.25, 0.75 and 1.5 mg IV nicotine; Experiment 2: 4 fMRI scan sessions administering two doses of 1.25 mg nicotine separated by 0.25, 0.75, 3 and 24 hours; Experiment 3: two injection series separated by 45 minutes, each injection series containing 5 rapid injections of 0.25 mg each, much like 5 puffs on a cigarette; Experiment 4: 3 fMRI sessions administering a single injection of 1.5 mg nicotine at 3 different rates- over 15, 60 or 120 seconds; and Experiment 5: 3 fMRI session administering 1.5 mg nicotine followed by exposure to either neutral, negative, or positive affect pictures. Since experimental arms are conducted in series, rather than in parallel, there are very few subjects common to more than one experimental arm. 