A radioimmunoassay was developed to measure the NH2-terminal counterpart of rat pro-atrial natriuretic factor (pro-ANF) in plasma. Synthetic rat ANF (Asp 11-Ala 37) coupled to bovine serum albumin was used to immunize New Zealand rabbits. The antiserum demonstrated good immunoreactivity towards rat ANF (Asn 1-Arg 98), (Asn 1-Tyr 126), (Asp 11-Ala 37) and even human ANF (Asn 1-Ser 30). The standard curve had an ED80 of 9.5 +/- 2.5 and ED50 of 44.0 +/- 10.5 fmol/tube. Immunoreactive ANF NH2-terminal peptide was measured directly in rat plasma without prior extraction. In fact, extraction of ANF NH2-terminal from plasma by C18 silica gel chromatography revealed inconsistent recovery and a lack of parallelism. Morphine (0.75 mg/100 g), chosen to elicit increased ANF (Ser 99-Tyr 126) secretion, elevated its plasma concentration from 54.1 +/- 3.2 to 190.8 +/- 55.8 fmol/ml after 20 min. At the same time, the immunoreactive NH2-terminal fragment rose from 378 +/- 16 to 1181 +/- 201 fmol/ml. The identity of this immunoreactive material was verified following affinity chromatography and reverse-phase high-performance liquid chromatography (HPLC) of plasma from morphine-treated rats. Molecular sieving and amino acid sequencing demonstrated that it appears to be consistent with or identical to rat ANF (Asn 1-Arg 98). The disappearance rate of ANF (Asn 1-Arg 98) was studied by injecting radioactive material into anesthetized rats. The exponential decay was analyzed by a two-compartment model in which the fast and slow components had a half-life of 2.5 +/- 0.3 and 54.8 +/- 3.9 min, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)