This study examined the factors that affect agrammatic sentence production by testing eight agrammatic aphasic participants' comprehension and production of active and passive sentences using two types of English psych verbs, those with an Experiencer-marked subject (Subject-Experiencer (SubExp)) and those with an Experiencer-marked object (Object-Experiencer (ObjExp)). The Argument Structure Complexity Hypothesis (ASCH, [J. Neuroling. 16 (2003) 151]) posits that the verb (and sentence) production difficulties observed in agrammatic aphasia can be attributed, at least in part, to the argument structure properties of verbs, with verbs that are marked for more complex argument structure (in terms of the number and type of arguments) presenting greater difficulty than those with less complex argument structure entries. Based on previous linguistic analyses of psych verbs, ObjExp psych verbs are more complex than SubjExp verbs. Therefore, we predicted that the former would present greater production (but not comprehension) difficulty than the latter. Results showed above chance comprehension of all sentence types, with the exception of SubjExp passive constructions, in which the subject position is occupied by a non-Experiencer argument. In active sentence production, ObjExp verbs were more impaired than SubjExp verbs. However, the opposite pattern was noted for passive sentence production. While all participants had difficulty producing passive sentences of both types, they showed better performance on ObjExp verbs, as compared to SubjExp verbs, in which the Experiencer is in the subject position. Further, agrammatic aphasic speakers showed a preference for producing actives for SubjExp verbs and passives for ObjExp verbs, indicating that the thematic role requirements of selected verbs (e.g., Experiencer, Theme) influence production patterns, as they do in normal speakers. These data, as well as the error patterns seen in our patients, support the ASCH and suggest that sentence production, disrupted in agrammatic aphasia, is impaired to a greater extent when the argument structure properties of the verb increase in complexity.