SummaryNeutral-mctdlloendopeptiddse in maternal plasma rises progressively during pregnancy, but no further vdriation was detected during spontaneous or induced labour. The level of the enzyme in amniotic fluid decreascs from mid pregnancy to term. Cord plasma enzyme activity was only one third of that in maternal plasma.Neutral-metalloendopeptidase (NEP; EC 3.4.24,11) was initially assumed to be, like angioten-sin I-converting enzyme (ACE; 3.4.15.2), a peptidyl-dipeptidase because it cleaves enkephalins at the same site as ACE (Erdos et al., 1978). Although it was called a true 'enkephalinase' (Schwartz et al., 1981). in humans and in animals the enzyme is identical to a neutral endopeptidase of the kidney (Almenoff and Orlowski, 1983; Erdos et al., 1985) which like ACE, cleaves many biologically active peptides. including enkephalins, angiotensins, substance P and neurotensin.We wanted to establish the activity of this enzyme in maternal and cord plasma and in amniotic fluid during pregnancy and labour, in view of the importance of its substrates in the nociception and modulation of pain from uterine contractions. We also investigated NEP activity at different stages in spontaneous and oxytocin induced labour, because the pain threshold during labour seems to be related to the number and intensity of uterine contractions and it is a general clinical observation that oxytocin induced labour is often painful, suggesting a possible impairment of the anti-nociceptive system.