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Children with attention-deficit/hyperactivity disorder (ADHD) exhibit impairments in response inhibition. These impairments are ameliorated by modulating dopamine (DA) via the administration of rewards or stimulant medication such as methylphenidate (MPH). It is currently unclear whether intrinsic DA availability impacts these effects of dopaminergic modulation on response inhibition. Thus, we will estimate intrinsic DA availability using magnetic resonance-based assessments of basal ganglia and thalamic tissue iron in 36 medication-naïve children with ADHD and 29 typically developing (TD) children (8-12y) who underwent fMRI scans and completed standard and rewarded Go/No-Go tasks. Children with ADHD additionally participated in a double-blind, randomized, placebo-controlled, crossover MPH challenge. We will use linear regressions covarying for age and sex to determine whether there are group differences in brain tissue iron, and to examine the relationship between brain tissue iron and response inhibition performance and the relationship between brain tissue iron and responsivity to reward. Further, in the ADHD group, we will assess the relationship between brain tissue iron and responsivity to MPH. The proposed study will clarify the role of brain tissue iron, a measure associated with subcortical DA availability, in the cognitive effects of reward-and MPH-related dopaminergic modulation in children with ADHD and TD children.