The serotonin transporter (SERT) is the major determinant of serotonin inactivation following release at synapses, is the site of action for many antidepressant, and is also targeted by a number of psychostimulants including cocaine, methylphenidate, and MDMA "ecstasy". SERT is expressed in both the central nervous system and gastro intestinal system. The serotonergic system is know to modulate mood, emotion, sleep and appetite and thus is implicated in the control of numerous behavioral and physiological functions. Decreased serotonergic neurotrasmission has been proposed to play a key role in the etiology of depression. SERT immunohistochemistry is thus useful to study serotoninergic fibers and related disease.