This study will evaluate Short Interval Intracortical Inhibition (SICI) measured by pTMS as a marker of the hyperactive-impulsive dimension in 120 ADHD 7-12 years, medication-free children. This study will characterize the effects of a single dose of atomoxetine compared to placebo on cognitive correlates of SICI change. Participants will be randomized 2:1 to either atomoxetine or placebo. The study will also characterize the effects of four weeks of atomoxetine treatment on cortical inhibition and will correlate SICI change with clinical outcomes.