A RIA for human brain natriuretic peptide (BNP) was developed. Both BNP and atrial natriuretic peptide (ANP) were extracted from human plasma with Vycor glass powder (71% recovery for BNP). The assay had a minimum detection limit of 0.45 fmol/tube and an IC50 of 9 fmol/tube. The within-assay coefficients of variation were 11.4% at 4 pmol/L and 3.2% at 22 pmol/L, and the between-assay coefficient of variation was 11% at 24 pmol/L. There was no significant loss of immunoreactive (IR)-BNP in plasma samples stored at -80 C for 4 weeks. Low rates of labeled BNP and IR-BNP degradation occurred in EDTA plasma incubated at 37 C. The mean venous plasma IR-BNP (6.3 +/- 0.3 pmol/L) in normal subjects (n = 48) was significantly lower than plasma ANP (8.4 +/- 0.6 pmol/L). In contrast to ANP, IR-BNP did not increase when normotensive or hypertensive subjects changed from erect to supine posture. Markedly elevated levels were found in patients with congestive heart failure (mean IR-BNP, 87 +/- 11 pmol/L; ANP, 87 +/- 12 pmol/L; n = 35), recent myocardial infarction (mean IR-BNP, 60 +/- 9 pmol/L; ANP, 33 +/- 6 pmol/L; n = 7), and chronic renal failure. High pressure liquid chromatography of plasma extracts from heart failure subjects revealed both high (mol wt, 10,000) and low (mol wt, 4,000) mol wt IR-BNP. High mol wt BNP was the major component (mean ratio, 1.9:1) and was linearly correlated with low mol wt BNP (r = 0.99). HPLC of plasma extracts from three normal subjects receiving constant infusions of human BNP (2 pmol/kg.min) showed a single major peak eluting in the position of hBNP-32, with no evidence of high mol wt material. These results show that whereas marked elevations in BNP occur in circulatory disorders, a major (> 50%) and consistent contribution to immunoreactivity is due to precursor forms. Further, compared to ANP, there is no IR-BNP response to supine posture in normal and hypertensive subjects.