There is uncertainty concerning the extent to which the auditory streaming effect is a function of attentive or preattentive mechanisms. The mismatch negativity (MMN), which indexes preattentive acoustic processing, was used to probe whether the segregation associated with the streaming effect occurs preattentively. In Experiment 1, alternating high and low tones were presented at fast and slow paces while subjects ignored the stimuli. At the slow pace, tones were heard as alternating high and low pitches, and no MMN was elicited. At the fast pace a streaming effect was induced and an MMN was observed for the low stream, indicating a preattentive locus for the streaming effect. The high deviant did not elicit an MMN. MMNs were obtained to both the high and low deviants when the interval between the across-stream deviance was lengthened to more than 250 ms in Experiment 2, indicating that the MMN system is susceptible to processing constraints.