The effect of a moderate dose of ethanol on the processing of low probability task-relevant and task-irrelevant stimuli was investigated using event-related potentials (ERPs). Sixteen subjects received alcoholic and placebo beverages on alternate days. ERPs were recorded from 15 locations on the scalp. The subjects were asked to press a button upon detection of rare target stimuli embedded among frequent standard and rare "novel" stimuli. Ethanol 1) reduced the amplitude of P3 to novel stimuli, but not P3 to target stimuli, 2) did not affect the mismatch negativity, and 3) delayed P3 latency and reaction time independently. These results suggest that 1) the processing of rare task-irrelevant stimuli is more vulnerable to the effects of ethanol than is the processing of task-relevant stimuli, and 2) ethanol impacts stimulus evaluation time and response production stages of information processing.