A radioimmunoassay (RIA) to measure C-type natriuretic peptide (CNP) of a dogfish, Triakis scyllia, was established, and plasma and tissue levels of CNP were measured. Molecular forms of CNP in plasma and tissues were also examined using a combination of the RIA and cation-exchange high-performance liquid chromatography (HPLC). The antibody used in the assay cross-reacted with all forms of Triakis CNP, as well as eel CNP, against which the antibody was raised. The antibody exhibited no cross-reaction with any atrial, brain (B-type), and ventricular natriuretic peptides examined and showed a weak cross-reaction with porcine CNP. The detection limit of this assay was 0.8 fmol/tube of Triakis CNP-22 which was used as standard. The CNP level in the Triakis plasma was 1.97 +/- 0.38 pmol/ml (n = 5) which far exceeded the physiological levels of any natriuretic peptides in other species. Among various tissues examined, the highest concentration of CNP was measured in the atrium, followed by the ventricle, brain, and pituitary. Low levels were detected in the kidney, liver, and digestive tracts. HPLC analyses revealed that the major form of CNP in the brain was CNP-22, while it was proCNP (CNP-115) in the heart. In contrast to other species from teleosts to mammals thus far examined, the majority of CNP in dogfish plasma was prohormone instead of processed, mature forms.