A study focused on the nature and effect of constraints on the hypotheses that learners make about the meanings of words. Two experiments were conducted at a large midwestern university: the first, involving 68 undergraduate students divided randomly into two groups, tested taxonomic and durative constraints on nouns, and time of day and cessation constraints on verbs; and the second, involving 56 students, presented students in each of two groups an opportunity to assign meaning to a target word, either a noun or verb, in order to determine if those reading the noun would associate object properties with the target word and those reading the verb would associate information about the manner in which the action was performed. Results confirmed that persons possess implicit knowledge of constraints on possible word meaning, and that they apply this knowledge in tasks which represent important aspects of natural word-learning situations. (Five tables of data are included. Appendixes contain a list of constraints investigated and a sample text used in the second experiment.) (NH)