To examine relationships between children's exposure to and use of new or rare words during preschool years and their later performance on vocabulary-related measures, this study explored frequencies of rare word use in different conversational settings. The study also tracked the use of rare words by mothers and children and related those results to vocabulary test scores and the ability to give definitions of a set of common words when the children were 5 years old. Children eligible for Head Start programs were recruited for the study. Eighty-four low-income families were visited at home once each year when the children were 3 and 4 years old. During these visits each mother performed a variety of tasks with her child. These tasks were reading two books to her child (one provided by the researcher and one familiar to the child); eliciting a report from the child of an interesting recent event; and playing with toys provided by the researcher. All events were tape recorded. Each mother was also asked to record a mealtime conversation. Analysis of the transcribed recordings indicated that the largest proportion of rare words were used during the reading of the book familiar to the child and during everyday conversation at mealtimes. However, no correlation was found between the use of rare words in book readings and later vocabulary measures. (MM)