A patient with proven malignant carcinoid syndrome of at least 6 years duration is presented. The administration of nitrogen mustard intravenously (0.4 mg per kgm) apparently adequately controlled the spread of carcinoid tumor metastases and massive recurrent ascites for more than 4 years. The administration of tolazoline for 23 days, benzyl analog of serotonin for 25 days, bromlysergic acid for 180 days, reserpine for 5 days, chlorpheniramine for 4 days, and alpha-methyl DOPA for 14 days was either ineffective in controlling the symptoms associated with excessive circulating serotonin, resulted in exacerbation of clinical manifestations or caused undesirable side effects. While both chlorpromazine and cyproheptadine were clinically effective; cyproheptadine, administered in large doses (80 mg per day) for more than one year, effected a more marked reversal of clinical symptoms. Except for benzyl analog of serotonin and chlorpromazine, which decreased excretion, (the urinary excretion of the latter drug causes "quenching" of the nitroso reaction and consequent low urinary excretion values) none of these compounds appeared to influence the urinary excretion of 5-hydroxyindolacetic acid.