Context: The biological basis for the development of major depressive disorder (MDD) remains incompletely understood. Objective: To quantify brain serotonin (5-hydroxytryptamine [5-HT]) turnover in patients with MDD. Design: Patients with depression were studied both untreated and during administration of a selective serotonin reuptake inhibitor (SSRI) in an unblinded study of sequential design. Healthy volunteers were examined on only 1 occasion. Direct internal jugular venous blood sampling was used to directly quantify brain serotonin turnover. The effect of serotonin transporter (5-HTT) genotype on brain serotonin turnover was evaluated and the influence of SSRI therapy on serotonin turnover was investigated. Setting: Participants were recruited from the general community following media advertisement. Experimental procedures were performed in the research catheterization laboratory of a major training hospital and medical research institute. Participants: Studies were performed in 21 patients fulfilling the DSM-IV and International Statistical Classification of Diseases, 10th Revision diagnostic criteria for MDD and in 40 healthy volunteers. Interventions: Treatment for patients consisted of SSRI administration for approximately 12 weeks. Main Outcome Measures: Brain serotonin turnover before and after SSRI therapy. Results: Brain serotonin turnover was significantly elevated in unmedicated patients with MDD compared with healthy subjects (mean [SD] internal jugular venoarterial 5-hydroxyindoleacetic acid plasma concentration difference, 4.4 [4.3] vs 1.6 [2.4] nmol/L, respectively; P=.003). Analysis of the influence of the 5-HTT genotype in MDD indicated that carriage of the s allele compared with the l allele was associated with greater than a 2-fold increase in brain serotonin turnover (mean [SD] internal jugular venoarterial 5-hydroxyindoleacetic acid plasma concentration difference, 6.5 [4.7] vs 2.7 [2.9] nmol/L, respectively; P=.04). Following SSRI therapy, brain serotonin turnover was substantially reduced (mean [SD] internal jugular venoarterial 5-hydroxyindoleacetic acid plasma concentration difference, 6.0 [4.0] nmol/L prior to treatment vs 2.0 [3.3] nmol/L following therapy; P=.008). Conclusions: Brain serotonin turnover is elevated in unmedicated patients with MDD and is influenced by the 5-HTT genotype. The marked reduction in serotonin turn-over following SSRI treatment and the accompanying improvement in symptoms suggest that high brain serotonin turnover may be a biological substrate of MDD.