Recognition memory for words can decline, sometimes precipitously, over the course of the recognition test. Decrements of as much as 75% in d' have been observed from the first quarter of testing to the last. and decrements of 40%-50% are not uncommon. Proactive mechanisms, imperfectly understood, appear to underlie such declines: Processing early input renders late input less recognizable, and making early recognition judgments renders later judgments more difficult. At the same time, the processing of late-input words fails to inhibit the recognition of the words that preceded them.