Abuse of prescription opioid medications has increased dramatically in the U.S. during the past decade, as indicated by a variety of epidemiological sources. However, few studies have systematically examined the relative reinforcing effects of commonly abused opioid medications. The current inpatient study was designed to compare the effects of intravenously delivered fentanyl , oxycodone, morphine, buprenorphine and heroin in morphine-maintained heroin abusers. All of the participants received all of the drugs tested; drugs and doses were administered in non-systematic order.