The EEG was recorded during passive listening and active attending to tone pips of an oddball stimulus sequence, and the time course of event-related gamma band activity (30-60 Hz) was compared with event-related potential (ERP) components. In the conventional frequency range (0.1-4.3 Hz) mismatch negativity (MMN) was produced in the passive listening condition. Concurrent with MMN, the power of event-related gamma activity to the deviant stimulus was higher than gamma activity elicited by the preceding standard stimulus. The transition from the N2b to the P3 wave, produced during active attention, was associated with a decrease in gamma band power upon the deviant tone. The results show the differential generation of gamma band power depending on preattentive and attentive stimulus processing.