This research deals with behaviors that are part of opioid dependence. The purpose is to study the extent to which stress and other factors, including money and amount of work effort, affect opioid choice. Specifically, the investigators will examine the effects of three issues/factors. The first is how hard participants are willing to work to obtain an opioid drug; the second is how much opioid drug would participants choose instead of money; and the third factor is how much participant's opioid drug choices are influenced after they are administered the drugs yohimbine and hydrocortisone, both of which could produce stress-like symptoms.