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The insect myosuppressin LMS (pGlu-Asp-Val-Asp-His-Val-Phe-Leu-Arg-Phe-NH2) elicits potent stimulation of the release of the digestive enzyme alpha-amylase from cell suspensions of the stomach-digestive gland complex of the scallop Pecten maximus. The myosuppressins are members of the FMRFamide-like peptide superfamily, which immunocytochemical data confirm is present in the scallop. Structure-activity studies indicated that the two most critical residues for bioactivity are Arg and Phe. Bioactivity of the peptide can be maintained if the basic, aromatic residue His is replaced by another basic residue (Lys) and another aromatic residue (Trp), but not the aromatic Tyr, indicating a sensitivity to the introduction of a phenolic OH group. A restricted-conformation analogue containing a cyclopropyl-Ala residue in position 8 (Cpa-MS) demonstrates an ability to antagonize the amylase secretion activity of LMS at microM concentrations. This result provides evidence that the myosuppressins adopt a tight turn in the C-terminal tetrapeptide active core region while binding to the scallop digestive gland receptor. Cpa-MS may provide a useful tool to neuroendocrinologists studying in vitro and in vivo digestive processes in mollusks and other invertebrates.