Methylphenidate has been shown to decrease impulsive choice (increase choices of a larger more delayed reinforcer). The purpose of this study was to investigate 2 potential behavioral mechanisms of this effect: a drug-induced change in control by reinforcement delay (Experiment 1) and/or by reinforcement amount (Experiment 2). In Experiment 1, pigeons responded under a rapid-acquisition, concurrent-chains choice procedure involving delay to reinforcement; the option with the shorter delay varied unpredictably across sessions. The pigeons accurately tracked the shorter delay across sessions (i.e., a preference for the option with the shorter delay developed within each session). Methylphenidate selectively decreased sensitivity to reinforcement delay-it attenuated the acquisition of preference at doses that did not systematically affect bias or response rates. In Experiment 2, pigeons responded under a rapid-acquisition, concurrent-chains choice procedure involving reinforcement amount. The pigeons accurately tracked the option with the larger reinforcement amount across sessions. Methylphenidate selectively decreased sensitivity to reinforcement amount-it attenuated the acquisition of preference at doses that did not systematically affect bias or response rates. These data suggest that methylphenidate attenuates the degree to which the various reinforcement dimensions control choice, and that drug effects on impulsive choice depend upon the relative contributions of drug-induced changes in control of behavior by each relevant dimension. (PsycINFO Database Record