The purpose of this investigation was to determine whether content words are selectively highlighted through increased vowel duration in mothers' speech to young children. Fifteen mother-child dyads served as subjects. All children (ages 1:6-2:4) were female and had a mean length of utterance between 1.00 and 1.40 morphemes. Each mother was asked to read five experimental stories aloud to her child and to an adult. The vowel durations of both content words and function words in these stories were examined. Vowels in content words--but not function words--were significantly longer in child-directed than in adult-directed speech. It is suggested that this characteristic of child-directed speech might heighten young children's attention to content words and contribute to the exclusion of function words during the early stages of English language development.