SEROTONIN (= 5-hydroxytryptamine = enter -amine) has been demonstrated in carcinoids of the small intestine as well as in the blood and urine of patients suffering from this disease1. The symptoms of these conditions are ascribed to the release of serotonin to the blood and tissues2. They consist primarily in cutaneous vasomotor signs (flushing), tachycardia, asthma-like dyspnoea and hyperperistalsis. Right-sided heart disease with tricuspid and pulmonal valvular lesions are noted in some but not in all cases. In prolonged cases pulmonary stenosis appears to be a fairly constant sign. Firm connective-tissue masses have been found to envelop the pelvic organs, as well as the iliac and hypogastric arteries. Considerable masses of connective tissue have also been observed deep to the cardiac valves, and microscopic examination has revealed connective tissue proliferation3. Articular swelling has been observed2. Hedinger and Gloor3 mention the possibility that carcinoids and their metastases may secrete a substance stimulating connective-tissue formation.