Auditory average evoked responses (AERs) to clicks ranging from 50 to 80 dB were studied in 9 normal adults while awake and during sleep. AER amplitude tended to increase little from 50 to 80 dB in waking subjects but increased markedly in sleeping subjects during stages 3 and 4. Rapid eye movement (REM) and stage 1 sleep had small amplitude AER.S in comparison with other sleep stages, individuals who showed decreases in amplitude at high intensities while awake slept significantly longer during the experimental nights.