In two experiments involving present/absent sample matching, we tested whether the visual stimuli or differential sample behavior served as the basis for comparison choice. In both experiments, one group (FR/DRO) was required to peck the present sample and to refrain from pecking the absent sample (as typically occurs with fixed duration present/absent samples), and the other group (FR/FR) was required to peck both samples. In Experiment 1, the samples were a black dot on a white field (present) and the white field alone (absent). In Experiment 2, the samples were a yellow hue (present) and a dark response key (absent). In both experiments, divergent retention functions were found only for the FR/DRO group. These results suggest that, in nonhedonic present/absent sample matching, it is the behavior directed toward the present sample, rather than the visual stimulus itself, that serves as the basis for comparison choice.