“…The precise mediators are not known, however, a number of vasoactive substances have been suggested including the 21 amino acid peptide, endothelin (Howard, 1987;Wilkes et al, 1990). Endothelin (ET) has been shown to be a potent vasoconstrictor of the human isolated foetoplacental cotyledon (Wilkes et al, 1990) and foetoplacental blood vessels (MacLean et al, 1992) and has the potential to influence placental blood flow either directly or in concert with other vasoactive agents including thromboxane A2, prostaglandin F2., prostacyclin, nitric oxide and atrial natriuretic peptide (De Nucci et al, 1988;Hu et al, 1988;Rae et al, 1989;Wilkes et al, 1990;Cameron et al, 1991). Circulating ET levels have been shown to increase during pregnancy and labour (Mastrogiannis et al, 1991); however, conflicting evidence exists concerning the possible role of ET in the pathogenesis of pre-eclampsia.…”