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Studies involving shifts in magnitude of reward in appetitive learning are reviewed with respect to the occurrence or nonoccurrence of socalled "contrast effects" (CE). Such studies have typically investigated "successive CE" in instrumental conditioning or "simultaneous CE" in differential conditioning and selective learning. In both cases, the usual result has been the occurrence of "negative CE" in the absence of "positive CE." Current interpretations of shifts in reward magnitude are examined and appear to be inadequate to account for these data. An alternative interpretation is suggested. Original data supporting the implications of this interpretation with respect to shifts of delay of reward and percentage of reinforcement are also reported.

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