The brain's processing of synonymity and antonymy was explored by examining the cortical evoked responses to correct judgments that a test word was a synonym or an antonym of a standard word presented 1 sec previously. Each of five subjects judged 256 pairs of words in each of two sessions, The evoked response to the second word was averaged separately for synonym and antonym pairs. Presentation of each test word as a synonym or an antonym, the order of presentation of each pair, and the side of the "synonym" response key were counterbalanced within subjects. The difference between the averaged response to antonym test words and that to synonym test words differed biphasically over the interval 250·650 msec after the stimulus. The demonstration of an evoked response difference between synonyms and antonyms extends the applicability of evoked potentials from attributes of individual word meaning to the semantic relationships between words.In several areas of cognitive research, investigators have adopted a multivariate approach in studying phenomena. For example, letter perception has been studied using accuracy, latency, effect on concurrent processing tasks, and evoked potentials (Posner, 1978). While this broad approach has been applied to many problems in psycholinguistics, only a little of this work has used the cortical evoked potential as a dependent measure. Recently, several attempts have been made to use the evoked potential as an indicator of the comprehension of meaning. While this research is new and at a pioneering stage, the results are promising. The present study attempted to determine whether evoked potentials were sensitive to certain semantic relationships more complex than those that have previously been studied. If it could be shown that evoked potentials reflect comprehension in tasks typically studied in semantic memory, then psycholinguistics will have another tool with which to investigate processes underlying the comprehension of meaning.The average cortical evoked response has been shown