The hypothesis was tested that serotonin (5-HT) modulates 3,4-methylenedioxymethamphetamine (MDMA)-induced increase in dopamine synthesis. Rats were treated with the selective 5-HT2 receptor agonist (R)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (R-DOI), the selective serotonin releasing agent 5-methoxy-6-methyl-2-aminoindan (MMAI), amphetamine, MDMA, or a combination of amphetamine and R-DOI or MMAI, followed by the L-dihydroxyphenylalanine (DOPA) decarboxylase inhibitor 3-hydroxybenzylhydrazine (NSD-1015). Rats were killed 45 min after the first injection and striatal DOPA was determined. R-DOI, MMAI, or amphetamine alone did not increase DOPA accumulation. However, combination of amphetamine with either MMAI or R-DOI significantly increased DOPA accumulation. Multiple doses of the R-DOI and amphetamine combination did not decrease [3H]paroxetine binding sites at one week after killing. The results indicate that the dopamine synthesis increasing effect of MDMA depends both on 5-HT2 receptor stimulation and dopamine efflux.