Previously, we have shown that changes in pigeons' divided attention performance resulting from changes in relative reinforcement are well described by the generalized matching law. In the present experiment, we examined whether sensitivity of performance to variations in relative reinforcement would be dependent upon sample duration. Pigeons responded on a delayed matching-to-sample procedure with compound samples (color + line orientation) and element comparison stimuli (two colors or two line orientations). Relative reinforcement for accurate matches on the two types of comparison trials varied across conditions. Sample duration was short (i.e., 0.75 sec) for half of the trials in a session and longer (i.e., 2.25 sec) for the other half. Sensitivity of accuracy to changes in relative reinforcement was greater with the longer sample than with the shorter sample, suggesting that differential reinforcement alters the allocation of attending to the elements of compound stimuli. Continued examination of the applicability of well-established theories of goal-directed behavior to the allocation of attention may provide further insights into what is variously referred to as goal-directed, voluntary, endogenous, or top-down control of attention.