Three experiments have examined whether a whole-partial reversal effect is due to shift in reinforcement density across phases, between whole and partial reversal in both matching (or nonmatching) -to-sample discriminations using 12 different stimulus sets (Experiments 1 and 2) and three concurrent discriminations (Experiment 3). In Experiments 1 and 2, rats were trained on nonmatching (or matching) -to-sample discriminations and then either given reversal training on 12 stimulus sets (W), on 9

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... out of them (P-9), on 6 out of them (P-6), or 3 out of them (P-3). Group W reversed faster than the other three partial groups. Group P-3 reversed faster than Group P-6, which in turn reversed faster than Group P-9. In Experiment 3, rats were concurrently trained on three discrimination tasks and then either given reversal on a total of three discrimination tasks (W), on two out of them (P-2), or on one out of them (P-1). Group W reversed faster than Group P-1, which in turn reversed faster than Group P-2 after overtraining. These findings provide evidence that rats form functional classes, and that the whole-partial reversal effect is not due to shift in reinforcement density.