Questions concerning the role of input in the growth of syntactic skills have generated substantial debate within psychology and linguistics. The authors address these questions by investigating the effects of experimentally manipulated input on children's skill with the passive voice. The study involved 72 four-year-olds who listened to stories containing either a high proportion of passive voice sentences or a high proportion of active voice sentences. Following 10 story sessions, children's production and comprehension of passives were assessed. Intervention type affected performance--children who heard stories with passive sentences produced more passive constructions (and with fewer mistakes) and showed higher comprehension scores than children who heard stories with active sentences. Theoretical implications of these results for the understanding of the nature of syntactic skills and practical implications for the development of preschool materials are discussed.