In order to study the role of working memory in sentence formulation, we elicited errors of subject-verb agreement in spoken sentence completion, while speakers did or did not maintain an extrinsic memory load (a word list). We compared participants with low and high speaking spans (a measure of verbal working memory for sentence production). As in previous studies, agreement errors occurred more frequently for sentence fragments with a singular subject noun and a plural noun than for corresponding fragments in which both nouns were singular. Agreement errors also occurred more frequently when the fragment had a distributive interpretation, so that the conceptual number of the subject mismatched its grammatical number, than when the fragment was not distributive. Importantly, there were effects of memory span and of memory load, and these variables interacted: Load affected only low-span speakers. Distributivity did not interact with either load or span. These data establish that a pivotal syntactic planning process is affected by verbal working memory limitations. As such, they constrain existing proposals about the role of working memory in language production.