The significance of central noradrenergic, dopaminergic and serotonergic neural systems for the locomotor stimulant effects of methylphenidate was investigated in the rat. In order to study the role of brain catecholamines, rats were pretreated with reserpine (2.5 mg/kg) followed 24 hrs later by treatment with alpha-methyltyrosine (25 mg/kg) or U-14,624 (75 mg/kg), a dopamine-beta-hydroxylase inhibitor. In these experiments, methylphenidate stimulated motor activity was antagonized by alpha-methyltyrosine and enhanced after treatment with U-14,624, suggesting that release of newly synthesized dopamine is important to a locomotor stimulant action of methylphenidate. Evidence implicating brain serotonin in the actions of methylphenidate was obtained in rats pretreated with pargyline or p-chlorophenylalanine (PCPA). Administration of pargyline 1 hr prior to methylphenidate was found to reduce the locomotor activity induced by methylphenidate and this was antagonized by pretreatment with low doses of PCPA. Higher doses of PCPA caused a significant elevation of methylphenidate induced activity which could be reduced by 5-hydroxytryptophan. Destruction of serotonergic neurons with 5,7-dihydroxytryptamine also potentiated methylphenidate induced locomotion. These latter findings suggest that serotonergic fibers have an inhibitory function in brain. These results are discussed in relation to the possible mechanism by which methylphenidate may act in hyperkinesis.