A study investigated children's hypotheses about the meanings of novel verbs on the child's first exposure to the verb. The study focused on the properties (action, result, or instrument) ascribed to the verbs before any information was given about word meaning. Subjects were 3-year-olds, 5-year-olds, and adults. The stimuli were six sets of videotaped events, each matched with a novel verb. For each verb there were three training events in which two of the three properties of the verb remained constant while the other changed (i.e., for two verbs, the instrument varied; for two verbs, the result varied, and for two, the action varied). After the third training event for each verb, the subject was asked if the test event showed the same verb. Results suggest that this "fast mapping" is useful in characterizing the learning of verb meanings, but that the hypotheses that guide children's initial (default) mappings are still changing in the preschool years, and that important and hierarchical relationships between default values in verb properties might exist. (MSE)