Middle latency auditory evoked potentials can be rapidly obtained using a 40/second stimulous rate, taking advantage of response periodicity to accelerate the averaging procedure. This technique was used in 31 normally hearing adults for low frequency response evaluation. Tone pips at 250, 500, and 1000 Hz were employed. The average difference between the middle latency evoked potential threshold and the subjective threshold was 17.9 dB at 250 Hz, 14.5 dB at 500 Hz, and 13 dB at 1000 Hz. The waveform of the response and the latency and amplitude of its main peaks were also analyzed. Thresholds at 500 and 1000 Hz were also determined using brainstem evoked responses in 15 of the subjects. These differed, on average, by 30 dB from subjective thresholds. The 40 Hz MLR paradigm thus appears to be an accurate and rapid method for low frequency threshold evaluation in adults, but it is greatly influenced by the state of arousal. During sleep, response amplitude decreases and threshold increases by about 15 dB. This may be of great concern when interpreting the results in children, in whom low frequency threshold evaluation is of primary importance.