This 6-week inpatient study will enroll opioid-dependent individuals. Participants will be required to reside at the research clinic for the entire study. All participants will receive morphine maintenance treatment on a daily basis. Twice a week participants will take part in experimental challenge sessions in which they will be randomly assigned to receive varying doses of tramadol, naloxone, morphine, or placebo. These sessions will assess the ability of tramadol to suppress opioid withdrawal symptoms. Participants will complete performance tasks to measure psychomotor and cognitive functioning. Heart rate and blood pressure will be monitored throughout the experimental sessions. A specialized camera will also be used to assess pupillary response of the eyes. Questionnaires and self-reports will be completed to assess medication effects and withdrawal symptoms. Following the end of the study, all participants will be offered outpatient drug abuse treatment.