Abstract The molluscan neuropeptide FMRFamide (Phe-Met-Arg-Phe-NH 2), the opioid [Met]enkephalin, and two peptides analogous to both were tested on four isolated molluscan muscles. Both FMRFamide and [Met]enkephalin-Arg 6-Phe 7-NH 2 (YGG-FMRFamide) excited the ventricle of Macrocallista nimbosa and inhibited that of Lampsilis claibornensis . These same two peptides also caused contractures of the anterior byssus retractor muscle (ABRM) of Mytilus edulis and the radula protractor muscle of Busycon contrarium . The effect on the ABRM was a catch contracture, relaxed by 5-hydroxytryptamine. In all cases, the two peptides were equipotent and their actions were identical. Neither [Met]enkephalin, nor the naturally occurring opioid peptide [Met]enkephalin-Arg 6-Phe 7 (YGG-FMRF), affected these preparations. In conclusion, these tissues are specifically sensitive to the structure at the C-terminal of FMRFamide, but not to its N-terminal or that of the enkephalins; that is, FMRFamide receptors, but not opioid ones, are present. Nevertheless, the amino acid sequence common to [Met]enkephalin-Arg 6-Phe 7 and FMRFamide implies that these two peptides and their naturally occurring congeners are homologous molecules.