ABSTRACTEighteen hyperactive clinical responders to methylphenidate were taken off medication and tested in two sessions during which they received an acute dose of methylphenidate and placebo in counterbalanced order. Seventeen normal boys of comparable age were tested without drugs. For both samples, evoked potentials were recorded during a Continuous Performance Test. Normal children made fewer errors of commission and omission and displayed faster reaction times than hyperactives tested under placebo. In addition, under active attention, the late positive component (LPC) of placebo‐treated hyperactive boys' evoked responses exhibited smaller amplitude than normal controls'. Methylphenidate increased the amplitude of hyperactive patients' LPC and generally ameliorated their performance, especially commission errors and reaction times. The results confirmed previous findings of attentional disturbance in hyperactivity and normalization by methylphenidate of these patients' performance and electrophysiological activity during sustained attentio