Twenty‐two normal young men received a counterbalanced and double blind administration of 20 mg methylphenidate and placebo. Two tasks were administered in counterbalanced order: an uninterrupted 45‐min vigilance test and a paired‐associates learning test. As previously reported, under placebo, accuracy and speed decreased monotonically over the course of the vigil. This degradation of performance was significantly reduced by the stimulant drug. Event‐related potentials (ERPs) were derived for correctly detected targets and nontargets. The most salient pharmacologic effects involved a late positive wave (P464) identified as P3b. Methylphenidate reversed or reduced the decrease in amplitude and increase in latency of P464 present over phases of the placebo session. The results suggest that the stimulant‐induced improvement in performance may be mediated by enhancement of evaluation processes. In the paired‐associates test there were no pharmacologic effects. Instead there was an unexpected, pronounced improvement in learning in the second session, irrespective of the substance administered. However, the ERP evoked by the “stimulus” cue of each pair was systematically related to the achievement of learning criteria. A late positive was identified as P3b (P555) increased significantly in amplitude following the attainment of learning. There were no amplitude changes among ERPs evoked in the early post‐criterion, late post‐criterion, or overlearning “phases.” Nor were there amplitude changes between early and late pre‐criterion categories. Thus, ERP amplitude displayed a binary association with the achievement of learning.

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