The production of serotonin (5-hydroxtryptamine) and certain related metabolites by tumours associated with the clinical carcinoid syndrome has now been well established. Many of the reported cases have concerned carcinoid tumours of the intestinal tract (Thorson, 1958 ; Macfarlane, Dalgliesh, Dutton, Lennox, Nyhus, and Smith, 1956) but tumours histologically resembling carcinoids, and also app...