Ten patients with metastatic carcinoid tumors in the liver showed elevated 5-hydroxytryptamine and substance P levels in plasma samples taken from hepatic or peripheral veins. Chromatographic characterization of the substance P-immunoreactivity showed that by gel permeation and high pressure liquid chromatography the substance P-immunoreactivity was indistinguishable from synthetic undecapeptide substance P. The results suggest that substance P, in addition to the 5-hydroxyinoles, may serve as a circulatory marker for some forms of carcinoid tumors.