Abstract.— Two experiments are described in which short-term “habituation” of the auditory evoked response (AER) was examined using discrete trains of click stimuli and averaging across trains. In the first study, the stimuli consisted of randomly interspersed trains of regular clicks, at 3 or 10 sec interstimulus interval (ISI). In the second study, the ISIs were also short or long but were irregular varying between 2.4 and 3.6 sec in the former case and 6 to 12 sec in the latter. The N1-P2 component of the AER diminished within the 3 sec trains especially for the regular stimulation; no such development occurred with the longer ISI. The total voltage in the background EEG and the reaction-times to the stimuli showed essentially similar changes. The possible mechanisms governing the response decrement are discussed and it is concluded that the data may be interpreted in terms of the Groves and Thompson dual-process theory of habituation.