Despite chloride being the bulk anion of the fetus little is known about mechanisms of maternofetal transfer by the syncytiotrophoblast of human placenta. In the studies reported in this thesis chloride transport by the human placenta has been investigated in three ways:- i) using isolated syncytiotrophoblast microvillous membrane vesicles ii) using the patch clamp technique to survey possible chloride channels which may contribute to the conductances identified on the microvillous membrane iii) using Northern blotting to look specifically for cystic fibrosis transmembrane regulator (CFTR) mRNA in trophoblast derived tissue. The vesicle experiments showed that the microvillous membrane of the syncytiotrophoblast contains an anion exchanger and two separate chloride conductances separable by the effects of inhibitors upon them; one is sensitive to diisothiocyano-2-2'-disulfonic stilbene (DIDS) and the other to diphenylamine-2-carboxylate (DPC). Patch clamp experiments on trophoblast derived cells in culture showed three distinct channels which appear to be chloride selective: (i) a large conductance channel (344-371pS) which may account for the DIDS-inhibitable conductance seen in vesicles. (ii) an intermediate conductance channel (20.3-22.7pS) (iii) a small conductance channel (8.8pS), seen only in the presence of PKA/ATP. It is not yet possible to determine whether one (or both) of the latter two channels accounts for the DIDS-insensitive conductance observed in vesicles. The small conductance channel has properties which would be consistent with it being CFTR but further characterisation is required for this to be certain. This small conductance channel was found in 27% and 33% of patches from mononucleated cytotrophoblast cells (derived from normal term placenta) cultured for 18hrs and 66hrs respectively but was not found at all in a choriocarcinoma derived cell line. This might reflect the effect of oncogenesis or of differentiation on expression of this channel. As in a previous report it was found in this study that whole placenta expresses CFTR mRNA but no signal was obtained from the cultured trophoblast derived cells; this may be because CFTR mRNA is expressed by non-trophoblast cells in the placenta or because its expression is at a relatively low level, below that detectable by the techniques used in this thesis. Further experiments will be required to determine which of these two possibilities is correct, including RNA protection assay, reverse transcriptase polymerase chain reaction or Western blotting for the CFTR protein itself. This study improves understanding of the nature of the transporters involved in chloride transport by the human placenta.