Long-latency components of event-related brain potentials (ERPs) recorded from subjects reading meaningful text are sensitive to semantic relationships among the major lexical items of sentences. In particular, the N400 components are enlarged to words that are semantically unrelated to or incongruous with the context provided by preceding items in a sentence. The present experiment was aimed at finding out whether this inverse relationship between N400 amplitude and semantic association would extend to situations where words were presented in isolated pairs, using a design that dissociated changes in N400 from confounding ERP waves elicited by active decision making. KRP's were recorded to 320 word pairs presented to eleven subjects. Each pair of words was followed by a letter, and subjects made a differential response according to whether or not the letter had been present in either of the words. After the ERP recording session, subjects rated the degree of semantic association between the words in each pair. ERP averages were formed on the basis of the subjects' ratings and on the basis of normative, a priori categories. For both types of averages the N400 amplitude was found to be a sensitive index of semantic association, even though the association was incidental to the subject's assigned task. These findings suggest the utility of the N400 measure in studies of semantic priming and as a probe of the automaticity of contextual influences in language processing.