During this stay they will participate in a total of 11 experimental sessions. Participants will take part in multiple trials in which they have the opportunity to choose drug, hydromorphone, or money. Hydromorphone is a heroin-like opioid. During the first two test sessions, participants will receive a sample of the drug doses that can be chosen. Before each of the final 9 test sessions begin, participants will be given a capsule containing either different doses of the drug yohimbine or a placebo (blank). Yohimbine is a drug that has been shown to produce a "stress"-like response in humans. Then participants will be given a capsule that contains either different doses of the drug hydrocortisone or a placebo (blank). Hydrocortisone is also a drug that can produce a "stress"-like response in humans. Then participants will have the opportunity to choose either drug or money by using a computer to earn choices. Respiration rate, oxygen saturation, heart rate, and blood pressure will be monitored throughout choice trials. Self-report questionnaires will be completed at different times during the study.