Abstract This study examined whether 5-hydroxyindoleacetic acid (5-HIAA) in cerebrospinal fluid (CSF) could be used to detect serotonergic damage induced by(±)-3,3-methylenedioxymethamphetamine (MDMA) in the central nervous system (CNS) of non-human primates. Monkeys were administered toxic doses of MDMA; two weeks later, the animals were lightly anesthetized with ether and CSF was obtained by means of cervical puncture. Later that same day, the animals were killed for direct determination of CNS serotonin and 5-HIAA concentrations. Monkeys with 73–94% depletions of serotonin and 5-HIAA in brain and 42—45% depletions of serotonin and 5-HIAA in the spinal cord had a60 ± 7% reduction of 5-HIAA in CSF, without any change in homovanillic acid (HVA) or 3-methoxy-4-hydroxyphenethyleneglycol (MHPG). These findings indicate that CSF 5-HIAA can be employed to detect central serotonergic damage produced by MDMA in non-human primates, and suggest that CSF 5-HIAA may be useful for detecting MDMA-induced neuronal damage in humans.