Recent studies have demonstrated that a preconditioning regimen (i.e., repeated low doses) of MDMA provides protection against the reductions in tissue concentrations of 5-HT and 5-HT transporter (SERT) density and/or expression produced by a subsequent binge regimen of MDMA. In the present study, the effects of preconditioning and binge treatment regimens of MDMA on SERT function were assessed by synaptosomal 5-HT uptake. Synaptosomal 5-HT uptake was reduced by 72% 7 days following the binge regimen (10 mg/kg, i.p. every 2 h for a total of 4 injections). In rats exposed to the preconditioning regimen of MDMA (daily treatment with 10 mg/kg for 4 days), the reduction in synaptosomal 5-HT uptake induced by a subsequent binge regimen was significantly less. Treatment with the preconditioning regimen alone resulted in a transient 46% reduction in 5-HT uptake that was evident 1 day, but not 7 days, following the last injection of MDMA. Furthermore, the preconditioning regimen of MDMA did not alter tissue concentrations of 5-HT, whereas the binge regimen of MDMA resulted in a long-term reduction of 40% of tissue 5-HT concentrations. The distribution of SERT immunoreactivity (ir) in membrane and endosomal fractions of the hippocampus also was evaluated following the preconditioning regimen of MDMA. There was no significant difference in the relative distribution of SERTir between these two compartments in control and preconditioned rats. The results demonstrate that SERT function is transiently reduced in response to a preconditioning regimen of MDMA, while long-term reductions in SERT function occur in response to a binge regimen of MDMA. Moreover, a preconditioning regimen of MDMA provides protection against the long-term reductions in SERT function evoked by a subsequent binge regimen of the drug. It is tempting to speculate that the neuroprotective effect of MDMA preconditioning results from a transient down-regulation in SERT function.