Evolutionary "old" nervous systems such as those of coelenterates are peptidergic: Using various radioimmunoassays we have now isolated 13 novel neuropeptides from sea anemones and several others from hydrozoan polyps and medusae. These peptides are all structurally related and contain the C-terminal sequence Arg-X-NH₂ or Lys-X-NH₂, where X is Ala, Asn, Ile, Phe, Pro or Trp. Three neuropeptides have a novel N-terminal L-3-phenyllactyl residue, which protects against degradation by nonspecific aminopeptidases. The neuropeptides from sea anemones are produced by different sets of neurones and have excitatory or inhibitory actions on isolated muscle preparations, suggesting that they are neurotransmitters or neuromodulators. We have also cloned the precursor protein for the sea-anemone neuropeptide Antho-RFamide (<Glu-Gly-Arg-Phe-NH₂). In Calliactis parasitica this precursor harbours 19 copies of immature Antho-RFamide (Gln-Gly-Arg-Phe-Gly) together with 7 other, putative neuropeptide sequences. The precursor of Anthopleura elegantissima contains 14 copies of Antho-RFamide and 19 other, putative neuropeptides. This shows that the biosynthetic machinery for neuropeptides in coelenterates, the lowest animal group having a nervous system, is already very efficient and similar to that of higher invertebrates, such as molluscs and insects, and vertebrates.