ERPs were recorded during a lexical decision task in order to investigate electrophysiological concomitants of semantic priming. The stimuli were 240 words and 240 nonwords presented one per trial at a fixed intertrial interval. Subjects were required to classify each stimulus as a word or nonword by pressing one of two response buttons. ERPs were recorded from 14 scalp locations, the right suborbital ridge, and the left earlobe, all referred to a balanced non-cephalic reference. RT and error data confirmed that semantic priming occurred under the conditions employed: primed words (those preceded by a semantically related word) were identified as words faster and more accurately than were unprimed words (those preceded by semantically unrelated words or nonwords). ERPs for all stimulus types were characterized by a large positivity peaking between 550 and 650 msec, preceded by a negative-going deflection peaking at approximately 400 msec. ERPs for primed and unprimed words were shown to differ significantly, diverging 200-250 msec following stimulus onset, reaching a maximum near the peak of the negative-going deflection at 400 msec. These differences were observed at locations over both hemispheres and were maximal in the centroparietal region. Although P300 latency differences between primed and unprimed words were also obtained, the priming effect on ERPs at shorter latencies could not be explained solely by P300 latency effects. Possible relationships between these ERP concomitants of semantic priming and P300, N200, and N400 were discussed.