This final report examines the pattern of biobehavioral responses to drowsy driving alarms and alerting stimuli. Following a standard overnight express run, commercial heavy-vehicle drivers completed an additional 4 hour run in a heavy-vehicle driving simulator. Previous research had validated the percentage of eyelid closure over the pupil over time (PERCLOS) as a generally useful and reliable biobehavioral index of lapses in visual attention (NHTSA Final Report, DOT HS 808 762). Results of the present study showed that the drivers anticipated the onset of the audible drowsiness alarm, from moment to moment, and initiated normal self-alerting behavior until the visual drowsiness indicator decreased. Driver response profiles revealed a pattern of increased general activation of alerting behavior when alarms were anticipated, as opposed to a pattern of predictable unit decreases in alertness following each alarm or alerting stimuli. Lane-keeping performance also improved in the feedback conditions during the return leg of the simulated run.