Seventeen children with attention deficit disorder (ADHD) and 10 normal controls performed two tasks while event-related potentials were recorded. ADHD subjects took part in two more sessions under methylphenidate (MP) or placebo. In the spatial orienting task, invalidly cued targets elicited a longer reaction time (RT) and a P3 that was longer in latency and greater in amplitude than did validly cued targets. Performance was similar for both groups, but the early portion of P3 (300-400 ms) was lower in amplitude for invalidly cued targets in ADHD subjects. MP increased accuracy without affecting RT and shortened P3 peak latency and increased the amplitude of its early portion. In the focused attention task, accuracy was greater for controls and MP, but there were no RT differences. Attended stimuli elicited greater amplitude P1, N1, and P3 than did nonattended stimuli, but these measures were unaffected by diagnosis or medication.