The presence of a vasopressor factor in clotted defibrinated blood has been known for 90 years. This factor, serotonin, was isolated from blood platelets and identified as 5-hydroxytryptamine in 1948, by Rapport. In 1937, enteramine, a smooth muscle constrictor which was later found to be identical with serotonin, was isolated from intestinal mucosa by Erspamer. He suggested that enteramine was a hormonal substance secreted by the argentaffin cells of the intestine. Intense interest in this entire subject was stimulated by recognition of the disease entity, malignant carcinoidosis. The syndrome is caused by malignant carcinoid tumors (argentaffinomas), which contain large quantities of serotonin. Patients with carcinoidosis have increased amounts of serotonin in the blood and large quantities of 5-hydroxyindoleacetic acid, the major metabolite of serotonin, in the urine. The clinical features of the carcinoid syndrome include episodic flushing of the skin, patchy cyanosis, telangiectasia, pellagra-like skin lesions, bronchoconstriction, edema, atypical valvular