 Smokers N420 will participate in one of 4 groups over a period of 24 to 52 days in sessions conducted in at the National Institute on Drug Abuse Intramural Research Program at 251 Bayview Blvd, Suite 200, Baltimore, MD 21224, and in a separately approved site, the Human Behavioral Pharmacology Laboratory HBPL, L484-496, 5841 S. Maryland Ave., Chicago, IL, 60637. We expect that 190 participants will successfully complete the withdrawal phase N50 in groups 1, 2, and 3a, N40 in group 3b. 