Procedures for generating arbitrary matching-to-sample performances may generate only conditional discriminations. Rational grounds for this distinction are proposed, based on the properties that any equivalence relation must possess. Empirical tests are described for determining whether subjects trained on conditional discriminations are also engaged in true matching to sample. A series of studies then leads to the conclusion that proof of true matching to sample by monkeys, pigeons, or baboons is yet to be provided. Whether the absence of such proof reflects experiential factors or species-defined limitations is not presently clear.Key words: conditional discrimination, matching to sample, stimulus equivalence, stimulus classes, stimulus control, key press, monkeys, baboons, children A commonly used experimental arrangement presents a subject with two discriminative stimuli simultaneously, perhaps a vertical and a horizontal line, while a third stimulus, perhaps a green or red hue, determines the positive or negative function of each line. Proce-

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