A vigilance study utilizing visual stimuli demonstrates one of the mechanisms of alcohol effects on human performance. The number of failures to respond to critical signals increases as a function of blood alcohol level. Using appropriate physiologic monitoring, it is possible to detect periods of drowiness with associated prolonged eyelid closures. Further insight into the mechanisms involved is gained when response failures are divided into eyes-open and eyes-closed failures. Alcohol produced a significant increase in eyes-closed (drowsy) failures but did not change eyes-open failures.