Cortical sources of neuromagnetic responses to noise bursts were compared in 7 healthy humans. The earliest response, P40m, peaking about 40 ms after the stimulus onset, was followed by a prominent deflection in the opposite direction at about 100 ms (N100m) and by another peak at 200 ms (P200m). A sustained field, seen near the end of the 400- to 550-ms stimuli, ended with an off-response of the same polarity as N100m. All deflections could be explained by cortical activity within the Sylvian fissure. The source of P200m was anterior to other sources of the on-response, except P40m. When noise bursts, or pauses of equal duration in a continuous noise were presented to the subject, striking similarities were found between the 100-ms deflections of the on- and off-responses: both peaked at about the same latency; their estimated sources were closed to each other in the supratemporal plane and their amplitudes depended in a similar way on the interstimulus interval (1.1-8.8 s). However, only the on-response was preceded by P40m, suggesting that P40m and N100m are not causally linked. N100m seems to reflect cortical activity related to any abrupt change in the auditory environment.