Grammatical inflections such as the English plural noun -s and third person singular verb -s are acquired at different points in time by young children. This finding is typically attributed to factors such as relative semantic salience or the distinction between lexical and functional categories. In this study input frequency, sentence position, and duration were examined as possible contributing factors. In both conversations with and stories aimed at young children, noun plural inflections were found to be more frequent than third singular verb inflections, especially in sentence-final position. Analysis of the speech of four mothers reading stories to their two-year-old children confirmed that duration differences also exist in the input. Because fricatives were lengthened in sentence-final position and plural nouns were much more likely to appear in these positions than were third singular verb forms, plural nouns were significantly longer than third singular inflections on average. The possible implications of these findings for language learnability theories and accounts of grammatical deficits in specific language impairment are discussed.