The malignant carcinoid syndrome is manifested by (1) paroxysmal flushing, (2) chronic diarrhea, (3) respiratory distress, and (4) valvular heart disease. These abnormalities result from a marked increase in the amount of circulating serotonin. The abnormal clinical findings in this syndrome result principally from the action of serotonin on the smooth muscle of the respiratory tract, the gastrointestinal tract, and the arterioles. The pathogenesis of the cardiac lesions remians obscure. Serotonin, or 5-hydroxytryptamine, is produced by the argentaffin cells of the gastrointestinal tract as a normal metabolic product of tryptophan metabolism. Tumors of the argentaffin cells—"carcinoids"—may produce tremendous amounts of serotonin, but since the liver metabolizes serotonin to "inactive" 5-hydroxyindoleacetic acid, symptoms of hyperserotonemia result only when serotonin in large amounts is present in the systemic circulation. Symptoms of the malignant carcinoma syndrome follow hepatic metastases, metastases elsewhere drained by the systemic veins, or from primary carcinoids outside the