Dopaminergic (DA) synaptic boutons were identified in rat striatum with an electron microscopic histochemical method. In rats which developed behavioral hypersensitivity after treatment with methamphetamine (MAP) for about 2 weeks, significantly fewer DA boutons were found. This effect was specific to DA boutons without mitochondria and was not seen in boutons with mitochondria. The density of granular synaptic vesicles in DA boutons, however, did not change significantly. These morphological changes would represent a high capability of nerve cells to reorganize synaptic connections under altered chemical environments, but they could not be related uniquely to behavioral hypersensitivity, as similar effects were observed in rats treated with haloperidol which did not develop behavioral hypersensitivity. Peculiarly, the effects of MAP treatment on both behavior and DA boutons were prevented by combined administration of haloperidol.