Fifty-seven kinds of APGWamide-related peptides and a peptide library consisting of 38 peptide mixtures, each of which contained 19 kinds of APGWamide-related peptides, were synthesized with a multipeptide synthesizer, and their APGWamide-agonistic or -antagonistic effects were examined on the anterior byssus retractor muscle of the bivalve Mytilus edulis and the crop of the land snail Euhadra congenita. The peptide mixtures having agonistic or antagonistic effects were subjected to HPLC purification to isolate the active peptides using the muscles as bioassay systems. Many peptides having agonistic or antagonistic effects were obtained. Of the antagonists, APGWGNamide, isolated from the peptide mixture of APGWGXamide, was the most potent. At 10(-4) M, APGWGNamide almost completely blocked the actions of 10(-6) M APGWamide on the anterior byssus retractor muscle of M. edulis and the crop of E. congenita.