Acute effects of alcohol and diazepam on reaction time (RT) and event-related potential (ERP) measures were examined in 108 healthy male volunteers. The subjects engaged in a simple RT task at two levels of stimulus intensity during baseline and treatment sessions. Lower stimulus intensity produced increased RT's, increased ERP peak latencies, and suppression of peak amplitudes. Moderate and high doses of alcohol, and high doses of diazepam produced increased RT's. Alcohol suppressed P100 and N100 amplitudes, while diazepam suppressed P100 amplitudes only. P100 amplitudes were correlated to RT's under baseline and treatment conditions. These results were taken as evidence for impaired stimulus detection during alcohol and diazepam intoxication, with both drugs influencing sensory-perceptual processes and alcohol alone influencing the degree of attentiveness.