After receiving methylphenidate HCl, rats conditioned for self-stimulation pressed a bar for stimulus at rates far exceeding those of similarly conditioned nonmedicated rats. Conditioned rats pressed an inactive bar more frequently than did nonconditioned rats, and this rate was definitely increased for conditioned rats which received methylphenidate. This drug did not impair the selection of an active from an inactive bar, but after stimulation considerably less selective ability was shown. This lack of discriminating action was probably due to the effects of repetitive reinforcement by stimulation rather than to the direct effect of the drug.
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