We evaluated [(11)C]fluvoxamine as a tracer for the serotonin re-uptake site. Studies of the distribution of the tracer in rat and primate brain showed adequate uptake of [(11)C]fluvoxamine, but failed to reveal regions with known high density of serotoninergic re-uptake sites. Pretreatment with unlabeled fluvoxamine did not substantially change the distribution. In rat brain tissue, nearly all radioactivity represented intact [(11)C]fluvoxamine. [(11)C]Fluvoxamine does not function as a tracer for serotonin re-uptake sites, owing to high nonspecific binding in the brain.