1. 1. Studies on nineteen patients with metastatic carcinoid are presented. 2. 2. Additional case reports illustrate the variability of clinical manifestations associated with carcinoid. It is now clear that some patients with extensive metastatic carcinoid do not show all, or even the predominant manifestations of the “carcinoid syndrome.” 3. 3. Chemical analyses confirm previous findings of excess serotonin production in this disorder. Also, the findings of low fasting plasma tryptophan and urinary N′-methylnicotinamide in some patients substantiate previous suggestions of a disorder in tryptophan metabolism in this condition. 4. 4. A tracer study, in one patient, with the serotonin precursor, 5-hydroxytryptophan, enabled calculation of the tumor pool of serotonin (2,800 mg.), its turnover rate (one-half life of five and one-half days), and the tumor mass (between 1 and 3 kg.). 5. 5. Increases in the urinary excretion of 5-hydroxyindoleacetic acid (5HIAA) in a patient during severe flushing episodes suggests that the flushes are mediated by increased serotonin release even though no concomitant rise in blood serotonin could be measured. 6. 6. Cardiac catheterization studies revealed no measurable differences in the serotonin content of mixed venous and arterial blood which, if present, might account for predominant rightheart involvement. It is suggested that if a pulmonary arteriovenous serotonin difference does exist, it is in that portion which is free in the plasma. Difficulties in determining plasma serotonin are discussed. 7. 7. The absence of serotonin in cerebrospinal fluid and the essentially negative results of psychologic evaluation in five patients probably indicate that serotonin does not penetrate readily into the central nervous system. 8. 8. With the possible exception of chlorpromazine, drug therapy has proved ineffective.