Occasional patients with medullary carcinoma of the thyroid (Multiple Endocrine Neoplasia Type II [MEN II]) are reported to have excessive serotonin (5-HT) production from the MCT; almost all patients with metastatic MCT have elevations in plasma concentration of the amine oxidase, histaminase. The elevated 5-HT production is thought to contribute to the troublesome diarrhea experienced by patients with MEN II. We compared the urinary excretion of 5-hydroxyindoleacetic acid (5-HIAA), the principle metabolite of 5-HT, of 33 patients with MCT with the urinary excretion of 5-HIAA in 33 control subjects. Six of the 33 MCT patients (18%) had severe diarrhea. The 5-HIAA excretion of the MCT patients did not differ from that of normal subjects. We also compared the platelet monoamine oxidase (MAO) activity of 27 MCT patients and 27 control subjects. The platelet MAO activity of the two groups did not differ. The 5-HT content and MAO activity of 6 of the MCTs was similar to normal thyroid tissue. The MAO activity of two follicular adenomas of the thyroid was greater than the MAO activity of MCTs. In contrast to the uniform elevation of plasma histaminase in patients with MCT, the platelet MAO activity is not altered and the majority of MCTs do not produce excessive amounts of 5-HT.