The present study investigated the effect of professional development (PD) on preschool teachers' conversational responsivity in the classroom, defined as teachers' use of strategies to promote children's participation in extended conversational exchanges (communication-facilitating strategies) and exposure to advanced linguistic models (language-developing strategies), and the resultant impact on proximal child language outcomes. We randomly-assigned 49 preschool teachers to receive 15-20h of such PD (PD; n=25) or to a comparison condition (n=24). Growth curve analysis indicated that trained teachers used significantly more communication-facilitating strategies across the year but no such difference for language-developing strategies. Moreover, children in these classrooms showed greater linguistic productivity and complexity in their talk. These findings suggest that PD may alter some aspects of teachers' conservational responsivity responsible for increasing the amount and complexity of child language. Alteration of some strategies, however, may require more intensive PD efforts. (Contains 6 tables and 2 figures.)