The two purposes of this study were (1) to explore the validity of certain indexes used to measure children's development toward maturity in the control of English syntax, and (2) to examine the characteristic exploitation of syntactic resources (a) by boys and girls, (b) at various age-grade levels, and (c) in speech and writing. The normative data was collected from taped oral responses or written language samples of 180 white middle class children, 30 each from kindergarten and grades 1, 2, 3, 5, and 7. Analysis was based on the mean length of the T-unit (a single, independent predication with all its complements and modifiers), which Kellogg Hunt (1964) claimed to be a discriminating indicator of the degree of syntactic mastery in children's language production. This study supported Hunt's finding. T-units were further analyzed for Chomsky's "sentence combining transformations." Results indicated that mean word length of total language production increases with every advance in grade level. The most significant increases in speech mastery were found in grades 1 and 7, and in writing mastery in grade 5. In grade 3 speech was better than writing, but this reversed in grades 5 and 7. Boys were better speakers than girls in all but grade 5. Girls wrote better than boys in the middle but boys surpassed them in seventh grade. (MH)