Single oral doses of the central and peripheral nicotinic antagonist mecamylamine were administered to healthy young normal males in doses of 5, 10, and 20 mg in a placebo-controlled, double-blind study. The 20 mg dose caused a significant increase in errors in the learning condition of the Repeated Acquisition task, producing a slower acquisition curve. The lower doses produced less errors, but more than in the placebo condition. There was no effect of drug on the performance component (retrieval of previously learned information). On the recognition memory task, dose-related increases in false-alarms during the delay period were seen, with little effect on misses or hits. Reaction time measures suggested a dose-related slowing of RT on several tasks. Behavioral effects were minimal and physiologic measures were consistent with dose-related ganglionic blockade. We interpret these results to indicate that acute blockade of nicotinic receptor function can produce measurable and significant cognitive impairment, even in non-smoking normals.