The presence and biochemical characteristics of human brain natriuretic peptide (hBNP) in the amniotic fluid at various gestational ages were investigated. The hBNP-like immunoreactivity (hBNP-LI) levels in amniotic fluid, determined by RIA, were 118.7 +/- 57.6 pmol/L (mean +/- SEM; n = 5) and 107.7 +/- 8.7 pmol/L (n = 9) in the first and second trimesters of pregnancy, respectively; it was significantly decreased to 28.4 +/- 5.1 pmol/L (n = 9) in the third trimester. However, human atrial natriuretic peptide-like immunoreactivity (hANP-LI) was not detected (< 0.3 pmol/L) in any of these samples. Northern blot analysis demonstrated hBNP mRNA in human amnion tissue. Moreover, cultured amnion cells secreted a significant amount of hBNP-LI (100-200 fmol/10(6) cells/day), but not hANP-LI, into the culture medium. The synthesis of hBNP in cultured amnion cells was further confirmed using the polymerase chain reaction. High performance gel permeation chromatography of hBNP-LI extracted from human amniotic fluid and the culture medium of amnion cells revealed that the predominant molecular form of hBNP-LI in both samples was the hBNP precursor, with an approximate mol wt of 12 kilodaltons. These findings indicate that hBNP is present in the human amniotic fluid, and that amnion cells synthesize hBNP and secrete it into the amniotic cavity.