The biochemical origin of serotonin from tryptophan is described. Most of the serotonin produced in the body is derived from the gastrointestinal tract. Normally, only a small amount of tryptophan is metabolized to serotonin; the majority leads to the vitamin, nicotinic acid, as an end product. In carcinoid, there is a reversal of this situation such that a majority of tryptophan is converted to serotonin. In addition to producing a large excess of serotonin which is responsible for many of the symptoms of the carcinoid syndrome, there may then also occur a deficiency of nicotinic acid.