3,4-Methylenedioxymethamphetamine (MDMA, “Ecstasy”) compels mammalian serotonergic neurons to release serotonin (5-HT). In this study, MDMA altered synaptic transmission presynaptically by enhancing quantal release in two model glutamatergic synapses—the neuromuscular junction (NMJ) of the crayfish opener muscle, which is enhanced by exogenous 5-HT application, and the NMJ of a larval body wall muscle in Drosophila melanogaster, which is insensitive to exogenous 5-HT application. At the crayfish NMJ, MDMA mimicked the actions of 5-HT but only at a substantially higher concentration. At the Drosophila NMJ, MDMA altered synaptic transmission but not through a 5-HT receptor. Using simple invertebrate preparations, we have demonstrated an additional non-serotonergic mechanism of MDMA activity that has not yet been addressed in vertebrate systems and that may play an important role in understanding the mechanism of action for a commonly abused drug.