Potential participants will initially attend a 4- to 6-hour screening session. This will include review of medical and drug histories, standardized psychological questionnaires, a physical exam, and blood collection. Eligible participants will then be admitted to the inpatient clinic. During the first day of experiment, participants will receive instructions and information about the study procedures. They will also undergo a physical exam, an electrocardiogram, standardized psychological questionnaires, and blood and urine collection. A single test dose of cocaine will be administered intravenously to all participants to ensure safety with continuation in the study. Vital signs will be monitored hourly throughout the day.