Methylphenidate is a psychostimulant which inhibits the dopamine transporter and produces dopamine overflow in the striatum, similar to the effects of cocaine. Excessive dopamine action is often associated with changes in gene expression in dopamine-receptive neurons. Little is known about methylphenidate's effects on gene regulation. We investigated whether a methylphenidate treatment regimen known to produce behavioural changes would alter gene expression in the striatum. Using in situ hybridization histochemistry, we assessed the effects of acute and repeated methylphenidate treatment on the expression of immediate-early genes (c-fos, zif 268) and neuropeptides (dynorphin, substance P, enkephalin) in adolescent rats. Acute methylphenidate treatment (0-10 mg/kg, i.p.) produced a dose-dependent increase in the expression of c-fos and zif 268. These effects were most pronounced in the dorsal striatum at middle to caudal striatal levels, and were found for doses as low as 2 mg/kg. Repeated treatment with methylphenidate (10 mg/kg/day, 7 days) increased the expression of dynorphin, which was highly correlated with the acute immediate-early gene response across different striatal regions. Moreover, after repeated methylphenidate treatment, cocaine-induced expression of c-fos and zif 268, as well as of substance P, was significantly attenuated throughout the striatum. These effects of repeated methylphenidate treatment mirror those produced by repeated treatment with cocaine or other psychostimulants and are considered to reflect drug-induced neuroadaptations. Thus, our findings demonstrate that acute and repeated methylphenidate treatment can produce molecular alterations similar to other psychostimulants.