A highly specific radioimmunoassay was developed for N-terminal peptide of salmonid proopiocortin using a guinea pig antiserum to the chum salmon peptide (sNPP 1). Since sNPP I has no tyrosine residue nor free N-terminal amino group, a mixture of minor components of sNPP 1, which have extensions of H-Val-LysGly- and H-Lys-Gly- at the N-terminus, were iodinated by the lactoperoxidase method after incorporation of 3-(phydroxyphenyl)-propionate to the terminal amino groups. Plasma and pituitary extracts of several salmonid species showed parallel displacement to the standard hormone. Samples from carp, goldfish, tilapia, and eel, as well as the plasma of hypophysectomized rainbow trout, showed no crossreactivity. Proopiocortin-related hormones isolated from the chum salmon pituitary, including melanotropins, endorphins, corticotropin-like intermediate lobe peptides, and gonadotropin and prolactin showed negligible cross-reactivity. NPP contents in the pars intermedia of rainbow trout and chum salmon were 10 to 15 times greater than those in the pars distalis. Plasma levels of NPP in the mature chum salmon caught in the bay were about 50ng/ml. Plasma NPP levels in the mature chum salmon of both sexes decreased after transfer from seawater to fresh water. Plasma cortisol showed a concomitant change with NPP, thus supporting previous findings that NPP modulates corticotropin action on the trout interrenal.