The level of endothelin (ET)-1, a uterotonin, increases in amniotic fluid during labor. The known metallopeptidases include ET-converting enzyme (ECE), which converts inactive precursor to potent ET-1, and neutral endopeptidase (NEP), which inactivates ET-1. These enzymes are present in fetal membranes, and the aims of this study were to establish the protein expression of the enzymes within the amnion of human fetal membranes. Expressions were compared between amnions obtained before and after term labor using a Western blot analysis and enzyme-linked immunosorbent assay, respectively. The localization of these enzymes was determined using immunohistochemistry. The protein expression of the enzymes and output of bioactive ET-1 in human amnion epithelial cells (HAECs) and mesenchymal cells (HAMCs) were investigated with and without proinflammatory cytokines, oxytocin, and prostaglandin treatment. The effects of sphingosine-1-phosphate (S1P), a bioactive lipid, were also examined. The protein expression of ECE-1 was significantly increased (P < 0.01), whereas that of NEP was significantly decreased, followed by increased ET-1 (P < 0.01), in the amnion obtained after labor (P < 0.01). HAECs and HAMCs primarily expressed ECE-1 and NEP, respectively. The protein expression of ECE-1 was significantly induced (P < 0.01). However, the NEP levels were significantly reduced (P < 0.05) by treatment with TNFalpha and IL1beta followed by the 7.5-fold and 6.5-fold increase of ET-1 (P < 0.01), respectively, in the HAECs. ET-1 was increased 2-fold by S1P (P < 0.01). These results suggest that the altered expression of enzymes regulating the activity of ET-1 during parturition is controlled by inflammatory cytokines.