Weekly spontaneous imitations and spontaneous productions in the speech of two language-impaired males (3-4 years old) were compared over a 10-week period until MLU (mean length of utterance) approached or exceeded 2.0 morphemes. Although imitation was extensive in both boys' output, consistency of imitation over time differed for both and was associated with ongoing changes in their lexicons. Imitation of multi-word utterances was selective to a small set of words, and this strategy accounted for significantly greater lexical diversity in Ss' imitative speech compared to their spontaneous speech. One child's imitative single- and multi-word lexicon became predominatly spontaneous over successive weeks, as did his imitative use of four contrastive semantic-syntactic relations. The second child's lexical imitations showed a shift only for words used as single-word utterances. Moreover, he demonstrated spontaneous control of several categorical rules for combining words at the outset of the study. Imitation for this subject appeared to primarily serve the function of introducing new lexical items into previously-practiced combinatorial patterns in his speech. An appendix provides the taxonomy used for coding 14 semantic-syntactic relations in Ss' multi-word utterances. A reference list and seven figures are also included. (Author/CL)