Brain serotonin synthesis and metabolism (turnover), as indicated by CSF concentrations of 5-hydroxyindoleacetic acid (5-HIAA), may depend on plasma concentrations of the essential amino acid L-tryptophan (TRP). We investigated the biochemical effects of acute plasma TRP depletion (ATD) in normal volunteers undergoing a 36-h CSF collection via lumbar drain. Six subjects who were in good health were put on a low-TRP diet (160 mg/day) 24 h before lumbar puncture; this diet was continued for the first 22 h of the CSF collection. At hour 22, subjects ingested a TRP-deficient 15-amino acid drink shown previously to deplete plasma TRP. Total plasma TRP, free plasma TRP, and CSF TRP subsequently decreased 86.3, 86.5, and 92.3%, respectively. CSF 5-HIAA decreased by 32.8%. Plasma total and free TRP concentrations were both decreased at ∼2 h following ingestion of the TRP-free amino acid drink and were lowest ∼6 h after ATD; CSF TRP and 5-HIAA were decreased at 2.5 h and ∼4 h after ATD, respectively. CSF TRP was lowest 8.0 h later. CSF 5-HIAA continued to decrease 14 h after the TRP-deficient amino acid drink was given.