This study proposes to evaluate the effects of guanfacine on brain activation during fMRI in 12 children and adolescents ages 8 - 15 with ADHD treated with once-daily INTUNIV(TM) (guanfacine; GXR) extended release tablets and 12 ADHD subjects randomized to placebo treatment. Children will be comprehensively assessed using a variety of clinical and neuropsychological measures. They will be scanned at baseline while performing both the go/no-go task (a well validated task for measuring inhibitory control (Durston et al., 2002, 2003)) and the Stay Alert task - a new task designed to measure the arousal component of attention, which was used successfully in a recent fMRI study of guanfacine in healthy adults (Clerkin et al., 2009). They will then be treated with GXR or placebo for 6 - 8 weeks in accordance with titration and dosing strategies used in recent Phase III dose optimization trials (e.g., up to 4 mg/day), and re-scanned while performing the same two tasks. The fMRI scans will be conducted using a dedicated research 3.0 T Siemens scanner.