Two studies were conducted on the effects of methylphenidate (20 mg) on young adults' event-related potentials. Although this stimulant elevated heart rate in study 1 (n = 14), it failed to affect either performance or amplitude of the late positive component (LPC) obtained from two versions of the continuous performance test (CPT). Because performance on these tasks was nearly error-free, we conducted study 2 (n = 23) to test the hypothesis that methylphenidate enlarges LPC amplitude only in more challenging tests than those used in study 1. In study 2, although heart rate was again elevated by the 20 mg dose of methylphenidate, LPC amplitude and performance were again unaffected in the two tasks employed in study 1 or in an easy tone discrimination procedure. However, in a more difficult version of CPT (10% errors of omission), both accuracy and amplitude of the concurrently obtained LPC were increased by methylphenidate. Similarly, in a choice-reaction time test, the stimulant increased speed as well as CNV amplitude. Our interpretation of the results is based on the view that LPC reflects the number of attentional resources committed to a stimulus during the evaluation stage. Thus, increases in LPC amplitude by methylphenidate are obtained only in tasks in which the subject may profit from the recruitment of additional attentional resources.