Thirty-one rats were trained on a differential reinforcement of low rate schedule. After responding had stabilized, animals were injected with methylphenidate, twice weekly, presession. Methylphenidate produced dose-dependent increases in response rates and decreases in reinforcements. Repetition of these doses produced a reduced drug effect, and a third administration of the 10 mg/kg dose further reduced the drug effect. Subsequently, the effects of daily and intermittent administration were determined for this dose. Daily methylphenidate, pre-session, produced tolerance to the behavioral effects of methylphenidate and cross-tolerance to the amphetamines. Twice-weekly methylphenidate, pre-session, produced partial tolerance to methylphenidate and partial cross-tolerance to the amphetamines. Thus, periodic exposure to the behaviorally disruptive effects of a drug of the amphetamine class reduces the effects of subsequent exposure.

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