serotonin is a neurotransmitter. its chemical name is 5-hydroxytryptamine or 5-ht. it is derived from tryptophan. serotonin is found in all vertebrates, mainly in the gastrointestinal tract, blood platelets and central nervous system. its name is derived from its effect on blood pressure: serotonin is a part of the "serum" which regulates the "tonus" of blood vessels.