Hypotheses concerning the cause of reduced serotonin transporter (SERT) density after MDMA administration were tested by comparing the effects of the tryptophan hydroxylase inhibitor p-chlorophenylalanine (serotonin synthesis inhibitor) and MDMA on serotonin transporter density in rat cortex and hippocampus. Rats were either treated with saline, 20 mg/kg MDMA twice daily for 4 consecutive days, or p-chlorophenylalanine (300 mg/kg i.p. for 2 days followed by 100 mg/kg on alternate days for 8 days after the second larger dose). Rats in each group were killed 1, 7 or 14 days after treatment, with Day 1 for p-chlorophenylalanine-treated animals considered the day immediately after the second dose of 300 mg p-chlorophenylalanine and Day 1 for MDMA treated animals being the day immediately after completing the MDMA dose regimen. Cortical and hippocampal SERT site density was assessed via binding assay with radiolabeled paroxetine and by autoradiography using Beta-CIT (referred to here as RTI-55). SERT site density was compared with cortical and hippocampal serotonin (5-HT) and its metabolite 5-HIAA, measured via high-performance liquid chromatography (HPLC). While both p-chlorophenylalanine and MDMA reduced 5-HT and 5-HIAA, only MDMA reduced SERT site density, as seen in both paroxetine and Beta-CIT assays. MDMA produced less reduction in 5-HT and 5-HIAA than did p-chlorophenylalanine, and positive associations were found between 5-HT levels and paroxetine binding in MDMA-treated animals on most, but not all, comparisons. (There was no relationship for hippocampal values on Day 14.) The authors interpret these findings as evidence that reduced SERT is indicative of MDMA neurotoxicity and not a neuroadaptive reduction in response to temporarily low stores of 5-HT. However, findings of reduced SERT binding 21 days after p-chlorophenylalanine (Linnet et al. 1995) indicate that inhibiting serotonin synthesis may also produce lower SERT sites, presumably through downregulation.