This study investigated the feasibility of a vocabulary-based teacher intervention to increase preschool teachers' production of complement clauses (e.g. "I wonder if we can put the monkey in the tree") in teacher-children play interactions. Using a multiple baseline across participants design we measured the impact of an intervention that promoted preschool teachers' production of four complement clause verbs (CCVs)--"figure out," "notice," "remember," "wonder." Based on visual analysis we established a functional relation between the teacher intervention and the teachers' production of the target CCVs produced with a complement clause. Asking teachers to increase their use of CCVs in verbal interactions with children is a promising approach to increase teacher complex syntax input in the preschool classroom.