All three experiments will involve inpatient hospital stays. Participants will be required to refrain from using any non-study drugs, other than nicotine, and will undergo daily urine and breathalyzer tests. Questionnaires will be used to assess drug effects and cocaine cravings, and a computerized test will measure motor function and performance. Heart rate and blood pressure will be monitored periodically; an electrocardiogram (ECG) will record continuously to monitor heart electrical activity. Following the end of each experiment, all participants will be offered a referral to a drug abuse treatment program.