Several types of contrast effects have been identified including incentive contrast, anticipatory contrast, and behavioral contrast. Clement, Feltus, Kaiser, and Zentall (2000) proposed a type of contrast that appears to be different from these others and called it within-trial contrast. In this form of contrast the relative value of a reinforcer depends on the events that occur immediately prior to the reinforcer. Reinforcers that follow relatively aversive events are preferred over those that follow less aversive events. In many cases the delay reduction hypothesis proposed by Fantino (1969) also can account for such effects. The current experiments provide a direct test of the delay reduction and contrast hypotheses by manipulating the schedule of reinforcement while holding trial duration constant. In Experiment 1, preference for fixed-interval (FI) versus differential-reinforcement-of-other-behavior (DRO) schedules of reinforcement was assessed. Some pigeons preferred one schedule over the other while others demonstrated a position (side) preference. Thus, no systematic preference was found. In Experiment 2, a simultaneous color discrimination followed the FI or DRO schedule, and following training, preference was assessed by presenting the two positive stimuli simultaneously. Consistent with the contrast hypothesis, pigeons showed a significant preference for the positive stimulus that in training had followed their less preferred schedule.Key words: within-trial contrast, contrast, stimulus preference, delay reduction, cognitive dissonance, key peck, pigeonsContrast refers to a comparison between two conditions in which the difference between them is amplified by the presence of the other. Three forms of contrast have been identified in the literature: incentive contrast, anticipatory contrast, and behavioral contrast.Incentive contrast occurs when there is a sudden, unexpected increase or decrease in reinforcement resulting in an overreaction to the change relative to a control group that experienced the final magnitude of reinforcement from the start. For example, Crespi (1942) trained rats to run for a large food reinforcer and then shifted them to a smaller reinforcer. Another group of rats was trained to run for a small food reinforcer and was then shifted to a larger amount of food. Running speed was compared to rats that had not experienced a shift in reinforcer magnitude. Rats that were shifted to smaller reinforcers ran slower than rats that had been trained to run for small reinforcers from the start (negative incentive contrast), and rats that were shifted to larger reinforcers ran faster than rats that had been trained with large reinforcers from the start (positive incentive contrast).A second form of contrast, anticipatory contrast, is observed when there are repeated predictable changes in reinforcer magnitude. Anticipatory contrast is assessed in anticipation of the change in reinforcement, and the dependent measure is typically a consummatory response rather than running speed. For exampl...