The effect of long-term stimulation on the transient 40-Hz response was studied in 5-h long experimental sessions. Ten human subjects were presented with series of 1000 Hz tone pips occasionally replaced by 1200 Hz tone pips. The transient 40-Hz response was compared with two event-related potential (ERP) components, the N1 and the mismatch negativity (MMN) during the different phases of the long-lasting sessions. The 40-Hz and N1 onset responses attenuated while the MMN did not. Thus, the 40-Hz response might indicate vigilance.