Twenty two college students of both sexes voluntarily participated in a study that evaluated the effect of discriminating second-order stimuli on acquisition and transfer of a conditional discrimination. Participants were assigned to six groups , exposed to different procedures and sequences. Three second-order matching-to-sample procedures were employed: comparison-stimulus selection, second-order stimuli selection, and stimulus-matrix selection. The results of this experiment show that the initial presentation of the stimulus-matrix procedure produced 100% correct performance from the first session. These results are examined in terms of the facilitating effects of the forced comparison of stimuli on the verbal discrimination of the matching criterion.