This paper explores "fast mapping", one of several processes that have been proposed to be involved in the rapid vocabulary expansion observed in the preschool years. An adaptation of a receptive word matching task examined how well children retained a just-mapped relation between word and referent when some information was later missing. Thirty-nine children between the ages of 3;0 and 5;6 (mean age 4;3) were taught to select a black square if the correct match for a spoken label was not visible in an array of pictures presented on a computer screen. This procedure allowed children to respond even when they perceived that the correct referent was absent. In experimental sessions, children experienced a single exposure to a word-referent relation. Then, under one condition they heard the just-learned label but were not presented with the matching referent; instead, a completely novel referent was visible along with the black square. Under another condition, they were presented with a just-learned referent (and the black square) but heard a completely new label. The question of interest was whether the children appreciated that an earlier-learned map precluded re-assigning a label to a new referent or re-assigning a referent with a second new label. If so, they should select the black square under both conditions. The majority of children (69%) resisted re-assigning a just-mapped label to a completely novel referent and selected the black square, even when the original referent was not in sight. However, fewer of these children resisted accepting a second label for a just-named referent (46%). Older children were significantly more likely to adhere to original maps than were younger children.