Healthy Caucasian male and female volunteers (N = 93), ages 18–35 years were recruited by posters, advertisement, and word-of-mouth referrals. To reduce variability related to tolerance and withdrawal from nicotine or caffeine, we excluded participants who reported smoking more than 10 cigarettes per week or consuming more than three cups of coffee per day. At screening, volunteers underwent a structured clinical psychiatric interview and a physical examination, including electrocardiogram (EKG). They also completed a psychiatric Symptom Checklist (SCL-90; Derogatis, 1983), the Michigan Alcoholism Screening Test (Selzer, 1971), and a health questionnaire with a detailed section on current and lifetime drug use and personality questionnaires including the ZKPQ (Zuckerman et al., 1993). They were excluded if they were taking medication for a current condition, if they had any current Axis I psychiatric disorder including substance dependence (in accordance with the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders [DSM–IV]; American Psychiatric Association, 1994), or any current or past medical condition considered to be a contraindication for d-amphetamine (e.g., abnormal EKG or hypertension). Candidates had to speak English and
