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Whether reward omission on the sample runs of a delayed alternation task would reduce proactive interference was tested in two experiments. On the first run of a trial, the animal entered one arm of a maze to obtain a food reward; on the second run the reward was available in the opposite arm. Alternation declined across a block of massed trials, but there was less of a decrease when the reward was omitted in the final trial or when another maze was substituted. In a task that required retention of three mazes at a time, alternation again declined across massed trials but was reinstated either by omitting the reward on the final trial or by substituting a different reinforcer. The results suggest that altering the reward outcome can improve memory discriminability of recently visited maze locations and thereby overcome trace confusion in memory.