Although some substituted amphetamines, like MDA, produce a combination of sympathomimetic stimulation and perceptual alterations, the psychoactive qualities of MDMA are less distinctive. MDMA binds to serotonergic receptors and has been shown to potently deplete brain serotonin concentrations. Biochemical and behavioral evidence suggests that MDMA may also act on the dopamine system. The present study explored the effects of blocking serotonin receptors on MDMA and amphetamine induced locomotor hyperactivity in rats. Locomotor activity was measured in photocell cages for 120 minutes following injection of methysergide (0, 2.5, 5, 10 mg/kg) or methysergide in combination with amphetamine (0.5 mg/kg) or MDMA (10 mg/kg). Methysergide, which had no effect on its own, significantly potentiated the locomotor hyperactivity produced by MDMA but not amphetamine. Thus, the intrinsic serotonergic agonist properties of MDMA may actually counteract the indirect sympathomimetic effects thought to be responsible for the locomotor hyperactivity MDMA produces.