FMRF-amide belongs to Molluscan Cardioexcitatory Neuropeptide. This peptide appears to localize with neuropeptide Y in some regions of the brain. FMRF-amide inhibits Na+-Ca2+ exchange in cardiac sarcolemma vesicles. The neuropeptide FMRF-amide stimulated enzyme activity 7- to 8-fold (EC50, 1 mM) via receptors that were pharmacologically distinct from those for dopamine and serotonin. FMRF- amide augmented cyclic AMP levels in slices of gill tissue with a time course similar to that for adenylate cyclase activation.