Norepinephrine is believed to modulate CNS processing of environmental signals. However, its specific role in stimulus evaluation processes has not been delineated. We examined the effects of the alpha2 noradrenergic agents, clonidine and yohimbine, on ERP and performance measures of auditory information processing. Ten healthy participants performed a three-tone target detection experiment, receiving either placebo, 0.2 mg clonidine, or 30 mg yohimbine, in a double-blind randomized design. The principal locus of action of the noradrenergic agents occurred between 100 and 200 ms poststimulus. P200 latency was sped by yohimbine and slowed by clonidine, and the frontal P3a was shifted in tandem. Components related to target detection (N250 and P3b) were unaffected. The results suggest that norepinephrine modulates CNS mechanisms of selective attention to infrequent stimuli. This may be relevant for patients with schizophrenia, a subset of whom exhibit selective abnormalities of these same ERP components. Our results offer a possible link between these two sets of findings, suggesting that some patients with schizophrenia may have dysfunctional noradrenergic systems.