Event-related potentials were measured as subjects read sentences presented word by word. A small N400 and a robust P600 effect were elicited by verbs that assigned the thematic role of Agent to their preceding noun-phrase argument when this argument was inanimate in nature. The amplitude of the P600, but not the N400, was modulated by the transitivity of the critical verbs and by plausibility ratings of passivised versions of these sentences (reflecting the fit between the critical verb and the inanimate noun-phrase as the verb's Theme). The P600 was similar in scalp distribution although smaller in amplitude, than that elicited by verbs with morphosyntactic violations. Pragmatically unlikely verbs that did not violate thematic constraints elicited a larger N400 but no P600 effect. These findings support the theory that the cost of syntactic processing on a verb is influenced by the precise thematic relationships between that verb and its preceding arguments. (Contains 20 footnotes, 11 tables, and 5 figures.)