The effect of processing load on event-related brain potentials (ERPs) was investigated in an intermodal selective attention task in which subjects attended selectively to auditory or visual stimuli. Processing load was manipulated by requiring subjects to detect either difficult-to-detect (deviant) or easy-to-detect (DEVIANT) targets in separate blocks of trials. Attention to auditory stimuli was associated with negative (Nda, 90-170 msec) and positive (Pda, 190-270 msec) enhancements in the ERPs to auditory stimuli. The Nda increased in amplitude with increasing processing load. Deviant auditory stimuli occurring among auditory standard stimuli elicited frontally distributed mismatch negativities (MMNs). The MMN persisted during visual attention and was unaffected by visual processing load. However, the MMN to deviants but not DEVIANTS was enhanced in amplitude with auditory attention. Attention to visual stimuli resulted in positive (Pdv, latency 70-130 msec) and negative (Ndv, 170-270 msec) modulations of visual ERPs, that increased with increasing processing load. Prominent visual deviance-related negativities were observed at occipital and infero-temporal scalp sites (latencies 90-290 msec), but only to DEVIANT visual stimuli. The early MMN-like portion of the visual deviance-related negativity was independent of attention, with equal amplitudes during different auditory and visual conditions.