Data on the complexity of relative clause formation in children indicate that right embedding precedes central embedding in development. Previous research on the subject argues that configurations where coreferential NP's function as subjects are less complex than configurations where coreferential NP's function as objects. It appears that the most elementary relative is one where the nominal subject of the embedded relative is equivalent to either the nominal object or the nominal subject of the main clause. A proposed development sequence, where O is object and S is subject, is OS, SS, OO, SO. A study to determine the prerequisites to relative clause formation in children aged 29 to 36 months, and to test previous research, was undertaken. Forty test sentences, 10 of each of the above types, were given to 10 children and imitation was elicited. Results generally supported prior research, and it might be proposed that the ontogeny of relative clause formation in part consists of a prerequisite ability to process conjoined simplex propositions with coreferential NP's functioning as subjects of the embedded relatives. (CK)