Auditory evoked potentials (EP) to high or moderate intensity, single or paired clicks were recorded from normal young adult subjects. A choice-reaction-time paradigm had two sets of instructions, for intensity discrimination and for number (single versus paired stimulus) discrimination. For intensity discrimination, the second click had no informative value and its N100 amplitude was markedly reduced relative to the first click. For number discrimination, the presence or absence of the second click provided the salient information, and N100 amplitude was actually slightly larger for the second compared to the first click. In contrast, the attentional manipulation had no effect on P50 amplitude, which showed over 50% suppression from the first to the second click for both tasks. Thus, suppression of P50 amplitude to the second of a pair of clicks is insensitive to attentional manipulations that have major effects on N100 amplitude. These findings suggest that abnormalities of schizophrenic P50 suppression reflect neuronal rather than psychological phenomena.