Coelenterate neurones produce peptides containing an Arg-Phe-NH₂(RF-amide)-like carboxyterminus. RF-amide-like peptides are located in neuronal dense-cored vesicles, indicating that they are released by exocytosis and that they might function as neurotransmitters or neurohormones. Using a radioimmunoassay for the sequence RF-amide, 3 peptides were isolated from the sea anemone Anthopleura elegantissima: < Glu-Gly-Arg-Phe-NH₂(Antho-RF-amide), <Glu-Ser-Leu-Arg-Trp-NH₂(Antho-RWamide I) and <Glu-Gly-Leu-Arg-Trp-NH₂(Antho-RW-amide II). The general structure of these peptides can be described as <Glu...Arg-X-NH₂, where X is an aromatic amino acid. From the hydromedusa Polyorchis penicillatus, the peptide <Glu-Leu-Leu-Gly-Gly-Arg-Phe-NH₂(Pol-RF-amide I) was isolated, which also belongs to the <Glu...Arg-X-NH₂ family. Using specific antisera, it was shown that all 4 peptides were located in neurones, many of which were associated with smooth muscle fibres. Application of low doses of Antho-RF-amide or of Antho-RW-amide I and II induced contractions of endodermal muscles of sea anemones. This suggests that these peptides are transmitters or modulators at neuromuscular junctions.