A 69-yr-old man with known carcinoid syndrome treated with octreotide and interferon-alpha 2b developed diarrhea, with six to eight watery to semiliquid stools per day. Diminished stool frequency and increased stool consistency were obtained by treatment with the 5-hydroxytryptamine-3 receptor antagonists ondansetron and tropisetron. Successful alleviation of the diarrhea was also observed with the alpha 2-receptor agonist clonidine. These observations indicate that these classes of drugs should be evaluated in a controlled trial in patients with carcinoid-associated diarrhea.