The Qualification Phase will be conducted as a single, 4-day visit. Doses will be administered in a randomized, double-blind crossover manner following administration of a standard low-fat meal. Subjects will be dosed with 20 mg of d-amphetamine or matching placebo d-amphetamine on Day 1 and Day 2, approximately 24 hours apart. PD assessments will be conducted before dosing and at time points for up to 8 hours post dosing. Safety assessments will be conducted before dosing and for at least 24 hours following dosing. Data will be reviewed to determine subject eligibility.