Intravenous application of ketamine is currently dramatically gaining in significance as a rapid and highly effective antidepressant treatment option. Ketamine modulates various neurotransmitter systems, though the mechanisms responsible for its antidepressant effects remain unkownn. However, the serotonin transporter (SERT) presents a target of high interest due to the SERT's fundamental role in depression's pathophysiology as well as in antidepressant response. The study at hand is the first to investigate ketamine's SERT binding in humans, by utilizing the highly selective SERT radioligand [11C]DASB and positron emission tomography. Further, investigation of severely depressed patients provides the unique opportunity to establish the relationship between ketamine's SERT binding and its antidepressant efficacy.