A series of cyclic peptides having the general structure H-Phe-c[-Nε-Lys-X-NH-(CH2)n-CO-] were designed on the basis of structure-activity relationship studies of motilin. All were motilin antagonists. The cyclic peptides, in which X is a 3-tert-butyl-substituted tyrosine residue (H-Phe-c[-Nε-Lys-Tyr(3-tBu)-βAla-] (3), H-Phe-c[-Nε-Lys-Tyr(3-tBu)-Gly-] (6), H-Phe-c[-Nε-Lys-Tyr(3-tBu)-Abu-] (7), and H-Phe-c[-Nε-Lys-Tyr(3-tBu)-Ahx-] (8)) showed potent motilin receptor antagonistic activity in the rabbit smooth muscle (pA2 > 7). The 3-tert-butyl Tyr was found to be the moiety responsible for enhanced binding to the motilin receptor, while the size of the ring had little importance.