This study assesses effects of buprenorphine (bup) and naloxone (nal) combinations in decreasing buprenorphine abuse liability. After stabilization, subjects will receive a series of 6 every other day challenge doses: 1) bup/nal placebo; 2)bup/nal (2:1); 3) bup/nal (4:1); 4) bup/nal (8:1); 5) morphine; 6) placebo. Ratings of opiate agonist and antagonist effects will be obtained. Pharmacologic effects of withdrawal following challenge doses are the main outcome measures.