Three analogues of tetragastrin, in which the tryptophan residue was substituted by 3-(1-naphthyl)-L-alanine, 3-(2-naphthyl)-L-alanine or 1, 2, 3, 4-tetrahydro-β-carboline-3-carboxylic acid, were synthesized and evaluated for their gastric juice stimulating activity. The results suggest that the indolyl NH function in the tryptophan residue plays an important role and that the role of the tryptophan residue in an interaction between tetragastrin and its receptor is different from that in the case of luteinizing hormonereleasing hormone.