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The two present experiments examined a premise of stimulus classes formation between discriminative stimuli in rats. Rats were concurrently trained on two discriminations and received occasional compound test trials on which both positive and negative stimuli were simultaneously presented . In Experiment 1, performance on tests was initially very poor, but it gradually improved as rats continued to receive overtraining on two discriminations. Overtraining on one discrimination alone was not sufficient to improve test performance. In Experiment 2, after receiving overtraining on two discriminations, rats were given extinction train ing either on both discriminations or only one before a final test. Extinction on both discriminations did not cause any loss on test performance, but extinction on only one discrimination did. Rats of the group that was extinguished on both discrimination tasks, evinced perfect discrimination performance on the test when food reinforcement was introduced again. These results show that each member of the stimulus class has both the unlearned representation of itself and the learned representation of the other stimulus during overtraining in two concurrent discriminations in rats.