Sperm respiration and motility of the sea urchin Hemicentrotus pulcherrimus were studied at pH 6.8 in the presence of a synthetic jelly peptide (Gly-Phe-Asp-Leu-Asn-Gly-Gly-Gly-Val-Gly) and monovalent cationic ionophores. The synthetic peptide stimulated sperm respiration and motility to the level of that found in normal sea water (pH 8.2) with half-maximal stimulation at approximately 100 pM. Monensin and valinomycin also stimulated sperm respiration with half-maximal effects at 7 μM and 0.7 μM, respectively. The stimulation of sperm respiration by the peptide and monensin was dependent on external Na+ , but was not dependent on the osmolarity of the suspending medium. Approximately 50 mM Na+ was required for half-maximal respiratory responses to the peptide and monensin.