This study of language development was intended to chart the developmental course of the spontaneous use of negatives and affirmatives by 1 1/2 - 3-year-olds in response to true or false statements concerning familiar objects, properties, and actions. Forty children, 18, 24, 30, and 36 months of age, were assessed for knowledge of the words used in the experiment and were then given a series of 48 statement verification tasks. Stimuli were simple existential statements of either true affirmatives ("That is a ball" about a ball), false affirmatives ("That is a car" about a ball), false negatives ("That is not a ball" about a ball), or true negatives ("That is not a car" about a ball). All sessions were videotaped and coded for affirmatives and negatives used by the children, for use of referent words (e.g. ball and car), and for nonverbal measures that distinguished comprehension strategies. Results suggested that by 2 1/2 years children manifest knowledge that language may fail to match reality. By 1 1/2 years, they distinguish appropriate true statements from other types. Relations in development between language as a communicative tool and as an object of cognition are discussed. (BD)