In an experiment conducted with 31 three-, four- and five-year-old Japanese children evidence was found for self-created definitions of the direct and indirect objects of verbs. Linguistic rules undergoing change during the course of the experiment were also observed. The results can be understood as showing that children are guided in their behavior by certain universals of language. Long before children master grammar they show the effects of systematic rules for the construction and comprehension of sentences. Sometimes these rules can be regarded as imperfect copies of adult rules. But often, when the two kinds of rules differ radically, the child's rules are better explained as being grammatical inventions, creations by the child which rest as much on his a priori expectations of the general form of language as on his experience with adult grammar. This experiment is one in which the effects of several such grammatical inventions are seen. The experiment focuses on a small point of grammar, the direct and indirect objects of verbs, but a number of rules are found that are unlike Japanese, the language to which the Ss are exposed, and like certain universals of language, in particular the universal distinction between marked and unmarked categories. (Author/KM)