This paper presents the results of two experiments which appear to show that children's linguistic generalizational biases change from a semantically based system to a syntactic-structural system. The experiments use the constituent repetition paradigm of Read & Schreiber (1982). Subjects were trained to repeat the subject noun phrase in orally presented sentences. Preschoolers, but not second-graders, displayed a tendency to repeat the agentive noun phrase (contained in the by-phrase) in semantically irreversible passive sentences. It is argued that the results provide more evidence for a semantic-relational bias in children's early grammars, and that the results also provide support for the notion that children's generalizational biases shift from a semantic relational basis to a syntactic-structural basis some time between the preschool and early grammar school years.