The automatic auditory feature analysis in the human brain was investigated using the mismatch-negativity (MMN) component of the event-related potential. MMNs were recorded in ignore and attend conditions to stimuli deviating from the repetitive standard stimuli simultaneously either in one, two or three features (frequency, intensity, stimulus-onset asynchrony). If the processing of each of these three features is independent of the others, the MMNs to the double and triple deviants should equal to the sum of the MMNs elicited by the corresponding single deviants. The double-deviant MMNs were found to be additive but the triple-deviant MMN was clearly underadditive. The results suggest complex interactions between brain processes involved in analyzing several simultaneous deviant features.