ABSTRACTAuditory average evoked responses (AERs) produced by tone onset (ON responses) and tone cessation (OFF responses) were studied in 14 normal adult subjects. When short (500 msec) tone bursts were presented widely spaced (2500 msec between tones), ON responses were large, in contrast to OFF responses, which were less than one‐third their size. But when long tones (2500 msec) were succeeded by brief (500 msec) silences, OFF and ON responses were comparable in size. In addition to this observed effect of the ratio of the percent of time the tone was on to the percent of time the tone was off, control experiments suggested that increased duration of preceding interval, unexpectancy of stimulus occurrence, and decreased mean frequency of stimulus presentation all increase the amplitude of both OFF and ON responses. OFF responses were found to be more sensitive to stimulus spacing effects than ON responses.