In Experiment I, pigeons were first trained on simultaneous matching-to-sample with either color stimuli or form stimuli, and then shifted to stimuli on the other dimension. Matching performance in the first session with stimtili on a given dimension was not affected by prior matching training with stimuli on the other dimension. However, in the first six color-matching sessions pooled, birds with prior form-matching training perfornme(d significantly better than birds without any prior matching training. In Experimient II, birds with experience matching both colors and forms in separate sessions were tested with novel stimulus configurations involving either novel stimuli or novel combinations of fanmiliar colors and forms. Matching performance was not affected by novel stimulus configurations, except that performance dropped to a chance level or below when the standard stimulus was novel. In Experiments II, III, and IV, three of four tests did not show any effect of prior reinforcement of pecks at a novel stimulus, presented alone, on subsequent matching of that stimulus. The results were interpreted as indicating that matching performance in pigeons depends on the learning of stimiulus-response chains involving the specific stimuli employed during training. An incidental observation in Experiments I and II was that there were typically more excess pecks at the standard stimulus during form-matching sessions than during color-matching sessions, which may be related to the fact that form matching is more difficult than color matching.In the matching-to-sample problem, the subject receives a reinforcer for choosing the comparison stimulus that is identical to the standard stimulus. What is learned by pigeons in solving the matching-to-sample problem?There are at least tlhree conceivable answers to this question. (1) Pigeons learn a generalized matching concept, which is independent of the specific set of stimuli employed during matching training. (2) Pigeons learn a set of stimulus-response chains based on the specific standard stimuli employed during matching training. For example, "peck red on the center key, then peck red on a side key," and "peck green on the center key, then peck green on a side key." (3) Pigeons learn a set of discriminations based on the specific stimulus configurations employed during matching training. A stimulus configuration is a specific set of stimuli and their specific spatial arrangement on