Nineteen boys with attention deficit hyperactivity disorder and thirteen normal control boys (ages 8-10) were compared to evaluate the effects of psychostimulant (methylphenidate, MPH) treatment on behavioral performance data and ERPs. The ERPs were recorded from 19 active electrode sites during a selective attention Stroop Test. The Stroop test did not discriminate baseline performance differences between ADHD boys and Controls; however, the performance measures were sensitive to MPH treatment effects. Several baseline ERP differences between the ADHD boys and Controls were identified for nontargets (P3a latencies) and targets (N1 latencies, P3b amplitudes and latencies). MPH affected both early and late nontarget and target ERP components evoked during the performance of the Stroop test which suggested that both early sensory mechanisms and later cognitive processes were affected by the psychostimulant. It is suggested that the results support attentional deficits in both anterior and posterior regions of the brain.