(+/-)3,4-Methylenedioxymethamphetamine (MDMA) was administered to rats as a single 40 mg/kg injection s.c. or 40 mg/kg s.c. every second day for 4 injections. Sixteen days following the last injection rats were killed. MDMA produced significant depletions of 5-HT and its metabolite 5-HIAA in the hippocampus and the frontal cortex. 5-HT was depleted to 30% of control value in the hippocampus following a single dose. 5-HT levels were not affected in the hypothalamus, suggesting differential effects on brain 5-HT systems. DA levels in the hypothalamus were significantly increased while NE levels in the frontal cortex were decreased to 73% of control following 4 doses of MDMA. MDMA, therefore, produces long-term depletions in 5-HT which suggests that it may act as a neurotoxin at 5-HT neurons in the brain of rats.