Buprenorphine is a mixed agonist-antagonist opioid that is being developed as a treatment for opioid dependence. Because buprenorphine is a partial mu agonist opioid, under certain conditions it is possible for buprenorphine to precipitate opioid withdrawal in opioid dependent individuals. A person with a high level of physical dependence could experience buprenorphine-related precipitated withdrawal. The purpose of this study is to test the acute effects of different doses of buprenorphine/naloxone in opioid dependent individuals maintained on methadone. The study is designed to provide dosing schedules similar to those that might be initially used in a physician's office.