Event-related brain potentials (ERPs) and autonomic nervous system (ANS) responses to occasional pitch and rise-time changes in a task-irrelevant auditory stimulus repeating at short intervals were measured while the subject performed a difficult intellectual task (Raven Matrices). It was found that deviant stimuli elicited the mismatch negativity (MMN) component of the ERP even when they elicited no ANS response. There was no significant difference in the mismatch negativity between trials in which the skin conductance response was or was not elicited. The pitch deviant tone also elicited heart rate deceleration, whereas the rise-time deviant tone tended to elicit a later heart rate acceleration. Neither heart rate change correlated with the mismatch negativity. The pattern of results obtained suggests that the mismatch negativity is generated by an automatic discrimination process associated with the cerebral events initiating the orienting response to stimulus change, but does not necessarily lead to the orienting response elicitation. Longer-latency ERP components tended to show slight covariation with ANS responses. The P3 was larger when a skin conductance response was elicited than when it was not elicited. Further, heart rate change trials tended to be accompanied by larger slow waves than trials with no heart rate response. Heart rate acceleration trials were accompanied by a larger slow parietal positivity and a smaller frontal negativity than were heart rate deceleration trials.