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1. The electrochemical gradients for Na+ and K+ were assessed in a cell culture model of trophoblast differentiation. 7. The Nae equilibrium potential (ENa) was positive in the trophoblast cells and the difference between ENa and Em was 122, 100 and 100 mV in JAr, mono-and multinucleate cytotrophoblast cells, respectively.8. These results suggest that the electrochemical gradient for K' is affected by the stage of trophoblast cell differentiation. In contrast, the electrochemical gradient for Na+ is similar in mono-and multinucleate cytotrophoblast cells.The transporting epithelium of the human placenta, the syncytiotrophoblast, plays an essential role in fetal growth and development. It is in direct contact with maternal blood and forms a continuous multinucleated cell layer which mediates materno-fetal exchange. Several ion transporters and conductances have been identified in its maternalfacing microvillous plasma membrane (e.g.Na+-H+ and Cl--HC03-exchangers; sodium-amino acid cotransporter; Cl-and K+ conductances) and fetal-facing basal plasma membrane (e.g. Na+-K+-ATPase and Ca2+-ATPase:reviewed by Shennan & Boyd, 1987;Sibley & Boyd, 1988;Shennan, 1992). However, it is unclear how these transport systems function in the intact syncytium to effect transplacental transfer and/or syncytiotrophoblast homeostasis.It is also unclear how their function changes during placental development and differentiation. The syncytiotrophoblast is formed throughout gestation by the fusion and differentiation of underlying cytotrophoblast cells (Boyd & Hamilton, 1970

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