The carcinoid syndrome can arise when effluent blood from carcinoid tumor tissue gains access to the systemic, as opposed to the portal, venous system. Features include facial flushing, diarrhea, wheezing, right-sided cardiac lesions, and retroperitoneal fibrosis. Attacks of flushing, diarrhea, and wheezing can be provoked by bolus injections of adrenaline, noradrenaline, or pentagastrin. While serotonin usually predominates, carcinoid tumors can also secrete, in varying proportions, 5-hydroxytryptophan, kallikrein, kinins, substance P and other neuropeptides, prostaglandins, catecholamines, and histamine. Of these, serotonin, kinins, histamine, and substance P are possible mediators of flushes; serotonin and substance P of hyperperistalsis; and serotonin, kinins, or histamine of bronchial constriction. Despite the gross excess of circulating serotonin, nearly all is platelet bound and therefore inactive. Very little is free in plasma. Demonstration of a contribution of serotonin to carcinoid attacks requires assay of free plasma serotonin; measurements of whole blood or serum serotonin are of little value. Some, but not all, provoked flushes have been shown to be accompanied by a rise in free plasma serotonin or substance P; an increase in circulating kinins has been more consistently shown. The 5HT2 antagonist ketanserin has been found to inhibit both provoked and spontaneous attacks of flushing, diarrhea, and dyspnea in a proportion of patients with carcinoid syndrome.(ABSTRACT TRUNCATED AT 250 WORDS)