Cerebrospinal fluid (CSF) levels of the monoamine metabolites 5-hydroxyindoleacetic acid (5-HIAA), 3-methoxy-4-hydroxyphenylglycol (MHPG), and homovanillic acid (HVA) were measured in three groups: 46 healthy volunteers; 9 medication-free patients with DSM III-R major depressive disorder, recurrent; and these same 9 patients following at least 4 weeks of fluoxetine treatment at 20 mg/day. CSF monoamine metabolite levels in medication-free patients did not differ from healthy volunteers; however, CSF 5-HIAA and MHPG decreased significantly from 95.9 +/- 24.6 (all values +/- SD) to 64.2 +/- 26.1 pmol/ml and from 46.7 +/- 14.2 to 42.6 +/- 11.6 pmol/ml, respectively, following fluoxetine treatment. Fluoxetine also significantly decreased mean Hamilton Depression Rating Scale scores from 23.2 +/- 6.5 to 17.4 +/- 5.0 and significantly increased the CSF HVA/5-HIAA ratio.