Plasma tryptophan (Trp) is reported to be decreased in some patients with the carcinoid syndrome. To determine if the plasma levels of other amino acids are also altered in the carcinoid syndrome, we used a fas-liquid chromatographic method to determine the plasma amino acid concentration of nine patients with the carcinoid syndrome and nine age-matched healthy control subjects. In comparison to the control subjects, the patients with the carcinoid syndrome had decreased plasma concentration of valine (Val), isoleucine (Ile), lysine (Lys), and ornithine (Orn), and an increased plasma concentration of methionine (Met). With the exception of a decrease in urinary excretion of proline (Pro) and hydroxyporline (Hyp), the patients with the carcinoid syndrome had normal quantities of amino acids in their urine. Plasma Met returned to normal when serotonin production by the tumor was reduced 60% by parachlorophenylalanine (PCPA); the other amino acid abnormalities persisted. Further studies are needed to determine the significance of these amino acid abnormalities.