Event-related potentials (ERPs) and behavioral responsiveness were investigated during the transition from wakefulness to sleep. Ten participants were presented with an auditory oddball task during repeated sleep onset periods. The EEG was recorded from 29 different scalp sites. A 1500-Hz tone pip was presented infrequently (p = .04) within a series of lower pitch 1000-Hz "standard" stimuli (p = .96). Participants were required to button press upon detection of the rare "target" stimulus. During wakefulness, almost all targets were detected. A large amplitude P300 was observed to these detected targets. This P300 was maximum over parietal areas of the scalp. During stage 1 sleep, subjects continued to respond on 47% of trials. The parietal P300 amplitude remained large to these detected targets. It was, however, much attenuated at frontal sites. When the participant failed to detect the target in either stage 1 or 2, no P300 was visible. P300 is thus associated with behavioral detection of the target stimulus, whether in wakefulness or stage 1 "sleep." Trials were also sorted by reaction time (RT), in which bins 1-3 represented increasingly long RT. In the waking state, P300 amplitude did not significantly vary across the different bins. Although mean RT latency varied by 512 ms from bin 1 to bin 3, P300 latency varied by only 25 ms. Differences in RT are thus probably due to response-related processes rather than stimulus classification time.