The sub-acute effects of single and repeated doses of MDMA on locus coeruleus neuron firing rate (LC), response to the alpha2 agonist clonidine, and on tyrosine hydroxylase activity, was investigated in rats. Rats either received saline, a single dose of 20 mg/kg MDMA, or a repeated-dose MDMA regimen (2 days of twice-daily 20 mg/kg MDMA given four h apart followed by 3 drug-free days, concluding with 2 more days of two 20 mg/kg MDMA injections). LC cell firing was measured via implanted electrode in anesthetized animals, and tyrosine hydroxylase activity was measured by assessing levels of 3,4-dihydroxyphenylalanine (DOPA) in anesthetized animals 30 min after inhibiting aromatic L-amino decarboxylase. Control and treated animals were then killed and assessed 18 h after MDMA (single dose, repeated dose) or 8 days later (repeated MDMA only). Finally, comparisons were made between rats given the serotonin synthesis inhibitor p-chlorophenylalanine (PCPA) alone, or after repeated-dose MDMA, with assessments made 18 h after MDMA. LC cell firing in single-dose or repeated-dose MDMA animals did not differ from controls 18 h after MDMA, and 8 d after MDMA, only repeated dose MDMA animals showed reduced LC firing. A single dose of MDMA did not attenuate clonidine-induced reduction in LC firing 18 h after MDMA, but clonidine was less effective in reducing LC neuron firing after repeated doses of MDMA. However, if rats were tested 8 d after repeated-dose regimen, clonidine again reduced LC firing. When measured 18 h post-drug, hippocampal NE and DOPA were not reduced after repeated-dose, but not single-dose, MDMA. When measured 8 d after repeated-dose MDMA, NE and DOPA values returned to normal. These findings suggest transient alteration in tyrosine hydroxylase activity. Reducing amount of brain serotonin (5-HT) through PCPA did not reduce hippocampal NE or DOPA levels. (Brain serotonin was reduced by up to 90% after PCPA). However, serotonin was also reduced after a combination of PCPA and repeated-dose MDMA and after MDMA alone. The authors hypothesize that serotonin and norepinephrine release lead to desensitization of alpha2 adrenergic receptors. The hypothesized effect is neither a residual effect of MDMA nor a long-term effect, as lower LC firing and changes in responsiveness to clonidine are present after estimated clearance of MDMA, but return to normal 8 days after drug administration. The changes in norepinephrine after repeated MDMA might arise because activation of alpha2 receptors leads to inhibited synthesis of DOPA. If these findings in rats given rather large doses of MDMA can be generalized to humans, they may indicate that at least some sub-acute effects of MDMA in humans are the result of transient alterations in NE system function.