Event-related potentials (ERPs) were studied in 10 practically healthy volunteers (6 male, 4 female) aged from 18 to 24 years during presentation of rare deviant and frequent standard visual stimuli in a standard odd-ball paradigm under conditions of active attention and inattention to stimulus duration. The deviation of the duration of deviant stimuli (50, 100 and 150 ms) from that of the standard stimuli (200 ms) varied from 50 to 150 ms. In series where deviations were 100 and 150 ms, a negative difference wave with the latency of 200-400 ms was revealed (ERP to the deviant stimulus under conditions of inattention minus ERP to the control stimulus of the same duration) mainly in the right temporal region. Since this difference negative wave appeared under conditions of inattention, we suppose that this wave is the visual analogue of mismatch negativity (MMN) and related to automatic detection of deviation of stimulus duration.