In the course of our screening program for cholecystokinin type-B (CCK-B) receptor antagonist, we isolated Q20547-A (1) from the cultured broth of Aspergillussp. Q20547. 1 inhibited CCK-8 binding to CCK-B receptors with an IC_<50> of 80nM. Structure of 1 was elucidated to a synmetrical dimer biosynthetically constructed from two molecules of L-Trp and L-Phe, and found to be identical with WIN64821, recently reported as a NK-1 antagonist. For the study of structure-activitiy relationship, we obtained many analogs of 1 by the biological mannar rather than complicated synthesis. First, weisolated six minor compounds analogous to 1 from the original cultured broth. Their structures were classified into two groups; one group consists of analogs with replacement one L-Phe unit of 1 to other amino acid (L-Leu, L-Val and L-Tyr), and the other group consists of stereoisomers or incorrectly biosynthesized isomers of 1. Second, we tried feeding experiments, various amino acids and non-amino acid substrates were added in the medium to obtain analogs of 1 biosynthetically. Many analogs of L-Trp and L-Phe were converted into the corresopnding analogs of 1, and a few amino acids (L-Leu, L-Val, L-met and L-thienylalanine) were substituted for L-Phe unit of 1. Furthermore, Trp-Xaa diketopiperaziznes were synthesized and fed to the culture. As expected from the biosynthetic passway, Trp-Xaa diketopiperazines were much better substrate than simple Xaa amino acids. Gly and L-Ala were incorporated instead of L-Phe by this method. Over 30 analogs of 1 obtained in these experiments were bioassayed. 23 and 25 (obtaind from feeding experiment of o-Cl-Phe and o-CF_3-Phe, respectively) were over 10-times more potent than 1. For the potent activity, comformation of four benzene ring seems to be important. Especially one phenylalanyl side chain is important to the binding, and the other phenylalanyl side chain is exchangeable for alkyl group of suitable size to show potent activity.