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Pigeons were exposed to a random relation between a conditioned stimulus (CS) and an unconditioned stimulus (US) under the signaled random procedure. All US presentations that were unpaired with the CS were preceded by a second cue, a cover stimulus. Consistent with prior studies, the signaled random procedure supported responding during the CS; however, magnitude of this cover-stimulus effect varied with the duration of the CS relative to the cover-stimulus duration. Higher levels of responding were maintained when the CS and cover stimulus were equal versus unequal in duration. Experiment 1 provides a systematic replication of a study by Williams (1994), who used rat subjects. Higher levels of CS-directed responding were maintained when the cover stimulus was equal in duration to the CS versus when it was shorter. Experiment 2 provides the first demonstration of the effects of cover-stimulus durations that are both longer and shorter than the CS duration, compared to a procedure in which the durations are equivalent. In support of a stimulus-duration-equality effect, level of responding was higher for all equal stimulus-duration groups in comparison to unequal stimulus-duration comparison groups. These effects were accommodated by a common-elements generalization approach to the Rescorla-Wagner model.