Methylphenidate hydrochloride (MP) is an effective treatment for attention-deficit/hyperactivity disorder (ADHD), a common neurobehavioral disorder of childhood onset characterized by inattention, hyperactivity, and distractibility. Methylphenidate hydrochloride blocks the dopamine transporters (DAT), the main mechanism for removing dopamine (DA) from the synapse, is believed to be involved in its therapeutic properties. However, the mechanism(s) by which increases in DA improve symptomatology in ADHD are not completely understood. Our studies of the dopaminergic effects of MP in the human brain using positron emission tomography (PET) have shown that MP blocks DAT, and that extracellular DA increases in proportion to the level of blockade and the rate of DA release (modulated by DA cell firing). These DA increases are greater when MP is given concomitantly with a salient stimulus than with a neutral stimulus, documenting the context dependency of MP effects. Additionally, MP-induced increases in DA are associated with an enhanced perception of the stimulus as salient. We postulate the MP's therapeutic effects are due in part to its ability to enhance the magnitude of DA increases induced by stimuli that by themselves generate weak responses, enhancing their saliency and the attention and interest they elicit. We postulate that these effects would improve school performance.