The hydromedusa Polyorchis penicillatus is a good model system to study neurotransmission in coelenterates. Using a radioimmunoassay for the peptide sequence Arg-Phe-NH2 (RFamide), two peptides have now been purified from acetic acid extracts of this medusa. The structure of one of these peptides was established as pyroGlu-Leu-Leu-Gly-Gly-Arg-Phe-NH2, and was named Pol-RFamide. This peptide belongs to the same peptide family as a recently isolated neuropeptide from sea anemones (pyroGlu-Gly-Arg-Phe-NH2). Using antisera to Pol-RFamide, the peptide was found to be exclusively localized in neurones of Polyorchis, among them neurones associated with smooth-muscle fibres. This suggests that Pol-RFamide might be a transmitter or modulator at neuromuscular junctions.