The encoding of temporal information is critical to auditory processing. Since the mismatch negativity component of the auditory event-related brain potential is thought to reflect properties of auditory sensory memory, we used it to examine the representation of acoustic time intervals in the human cortex. The mismatch negativity occurs in response to deviations in acoustic regularities, which are stored in sensory memory. We used 16 stimulus conditions, randomly presenting short trains of tones with fixed onset-to-onset intervals of 100, 200, 300 or 400 ms (all tones in the study were identical). The first four intervals between the tones established the acoustic regularity on each train (i.e. the ‘standard’). The fifth tone in each train was preceded by an interval that varied randomly among the same four intervals. If this interval was different from the standard for that trial, it violated the acoustic regularity (i.e. it was a ‘deviant’). The mismatch response to the fifth tone differed significantly among stimulus conditions and was proportional to the absolute value of the logarithm of the deviant/standard interval ratio. This indicates that short acoustic time intervals are represented with a ratio scale in the human cortex. When the fifth tone occurred 100 ms after the fourth, it elicited a somewhat different, although proportional response, supporting the hypothesis that a special integration mechanism may exist for very short time intervals.