In humans, deviant auditory stimuli elicit an event-related potential (ERP) component, termed "mismatch negativity" (MMN), that reflects the operation of a cortical detector of infrequent stimulus change. Epidural auditory ERPs were recorded from 3 cynomolgous monkeys in response to soft and loud clicks. "Oddball" loud or soft stimuli elicited a long-duration frontocentral negativity, peaking at approximately 85 msec, that was superimposed upon cortically generated obligatory ERP components. These data suggest that monkeys might serve as a heuristically valuable system in which to study the neurochemical and neuroanatomical substrates of early context-dependent ERP generation.