The purpose of this study is to determine if the opioid agonist effects of doses of 4/1 mg, 8/2 mg, and 16/4 mg of buprenorphine/naloxone can be completely blocked by 50 mg of oral naltrexone. This study will provide data to support the design of a similar study of the depot formulation of naltrexone and ultimately to study this combination for the treatment of cocaine dependence.