We recently characterized a novel heart-specific hormone from salmon (salmon cardiac peptide, sCP). We have now prepared a recombinant plasmid expressing the NH₂-terminal fragment of pro-sCP (NT-pro-sCP) and used it to set up a specific RIA for the peptide. Because of the sensitivity of the assay and the high circulating levels, NT-pro-sCP can be measured from as little as 2 µl of serum. This enables repeated sampling from the same animal in different experimental setups. Mechanical load increased the release of NT-pro-sCP from isolated perfused salmon ventricle, in parallel with sCP. Bolus injection of human endothelin-1 (ET-1; 1 µg) in the dorsal aorta of salmon resulted in an extensive increase of serum NT-pro-sCP (from 0.99 ± 0.11 to 4.6 ± 1.5 nmol/l). The response was abolished by pretreatment with a specific type A ET (ET_A) receptor antagonist (BQ-123) but not with a type B ET receptor antagonist (BQ-788).The NT-pro-sCP levels had a good correlation with those of sCP (r² = 0.75). Our results demonstrate the practical usefulness of circulating NT-pro-sCP as a marker of the endocrine function of salmon heart. They also suggest that ET-1 has an important role in regulating sCP release from teleost heart by an ET_A receptor-mediated mechanism.