To investigate the effects of heavy social drinking on sober cognitive processing, event-related potentials were recorded from 13 heavy social drinkers and 13 light social drinkers in the presence and absence of a pharmacological challenge (i.e., lorazepam). Event-related potentials were elicited by a task reuqiring continuous recognition memory for visually presented words. The heavy social drinkers exhibited shorter P2 latencies than the light in the task) versus “new” words. Lorazepam increased motor reaction time to correctly identified old words and produced a deficit in recognition memory only in the light social drinkers. Light social drinkers had an increased P300 latency anda larger P300 amplitude to new words in the lorazepam treatment. The differences in cognitive functioning evident between heavy and light social drinkers were reflected in event-related potential deviations and appear to indicate a tolerance in heavy social drinkers to the effects of lorazepam.