Modafinil is a novel wake-promoting drug used for the treatment of narcolepsy, the mechanism of action of which remains unclear. Previous studies have shown that modafinil produces a different pattern of c-Fos activation in the brain to the classical stimulants amphetamine and methylphenidate. Modafinil, given i.p. to urethane-anesthetized rats, is associated with an increase in histamine release from the anterior hypothalamus, indicating that its behavioral actions may involve histaminergic systems. In the present study, the effects of modafinil on histamine release using in vivo microdialysis and locomotor activity in freely moving rats were examined, and compared with those of the classical psychostimulant methylphenidate. Modafinil (75 and 150 mg/kg, i.p.) increased both histamine release and locomotor activity, significantly. Methylphenidate (3 mg/kg, i.p.) also increased locomotor activity to the same extent as modafinil (150 mg/kg, i.p.) without stimulating histamine release. Depletion of neuronal histamine using alpha-fluoromethylhistidine abolished the effect of modafinil on locomotor activity in mice but had no effect on methylphenidate-induced locomotion. Examination of the effects of modafinil and methylphenidate on locomotor activity in the dark phase at doses that produced comparable effects in the light phase showed that the effect of modafinil in the dark phase was less than that of methylphenidate, a possible indication that modafinil-induced locomotor activity may be partly related to its wake-promoting actions. These findings suggest that the locomotor effects of modafinil but not of methylphenidate, involve the central histaminergic systems.