Electrical activity was recorded from adult subjects during four consecutive nights of natural sleep. Ongoing EEG activity was recorded from an electrode on the vertex referred to electrodes on the right and left mastoids. An electrode placed near the outer canthus of each eye was referred to the left mastoid and used to record eye movements (EOG). The early components of the averaged electroencephalic response (AER) were examined during the third and fourth nights, with 50 dB SL clicks at a rate of 9.6/sec presented continuously from a loudspeaker during the entire night. Each average consisted of 1024 responses. Sleep stages were subsequently scored from the EEG and EOG, and AERs were selected for analysis based on predetermined sampling rules. The response during sleep differs only slightly in latency from that found in waking subjects. Latencies of the major peaks remain constant regardless of stage of sleep. Amplitude generally varies with stage of sleep: the deeper the stage of sleep, the smaller the amplitude. REM sleep does not interfere with responsivity and, in fact, may enhance clarity of response. In addition, overall response patterns from stages of sleep compared early in the night to those from the early morning show no evidence of long-term habituation during the course of the night.