In a randomized double-blind placebo-controlled crossover study, 17 hyperactives, clinically considered as drug responders, were administered a battery of information processing tasks to assess the efficacy of methylphenidate. The investigation concentrated on sustained attention, and, following a linear stage model of information processing (divided and focused attention), encoding, filtering, selective set and response organization operations. Methylphenidate did not affect short-term memory or visual retention, baseline motor speed and encoding. Methylphenidate improved vigilance aspects of sustained attention. The drug did not improve filtering but did enhance selective set (target search) operations. The decline in speed and speed variability as a result of medication could not readily be explained in terms of response organization processes. An attempt was made to unite the experimental results through the concept of signal-response frequency.