The influence of five programs of the Mt. Druitt Early Childhood Project on the development of preschool children's vocabulary and syntactical skills was assessed in a pretest/posttest design. Programs differed in the amount of structure they imposed on the language experiences of the children and in the content they specified. Different measures were selected for assessing children's use of vocabulary and basic and elaborated syntax. Vocabulary was measured by the Circus I Test: What Words Mean. Basic syntax was assessed in the areas of the correct use of pronouns, tenses, voice, and subject/object relationships. Experimenters used three methods of measuring children's control of syntactic structures: imitation, comprehension, and production. Speech samples were collected to assess children's use of elaborated syntax. A controlled situation was used so that direct comparisons could be made between children enrolled in different programs. Indexes chosen to represent ability in elaborated syntax were the Noun Complexity Index and the Verb Complexity Index. In addition, the mean length of utterance was computed. The posttest data revealed significant differences in scores obtained by children in different programs in both the vocabulary test and the comprehension subtest of the syntax test. The practices of the different programs did not influence the children's syntactical development as measured by imitation and production. (Author/RH)