We recently characterized a novel heart-specific hormone from salmon (salmon cardiac peptide, sCP). We have now prepared a recombinant plasmid expressing the N-terminal fragment of pro-sCP (NT-pro-sCP) and used it to set up a specific radioimmunoassay for the peptide. Due to 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 in vitro and in vivo experimental setups. We showed with the novel radioimmunoassay that mechanical load increases the release of NT-pro-sCP from isolated perfused salmon ventricle, in parallel with sCP. Bolus injection of human endothelin-1 (1 µg) into the dorsal aorta of salmon results in an extensive increase of serum NT-pro-sCP (from 0.99 ± 0.11 to 4.6 ± 1.5 nmol/l, P < 0.001). The response was abolished by pretreatment with a specific ETA receptor antagonist (BQ-123) but not with an ET_B receptor antagonist (BQ-788).The NT-pro-sCP levels had a good correlation with those of sCP (r² = 0.75).Thus NT-pro-sCP measurements can be used to estimate the cardiac release of the biologically active sCP. 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 endothelin-1 has an important role in regulating sCP release from teleost heart by an ET_A receptor mediated mechanism.