During pregnancy, the fetus is entirely dependent on its mother for nutrition. Most nutrients are transferred from the mother across the placenta. A series of transport mechanisms have evolved to carry out these tasks and there has been substantial effort in elucidating their regulation and modulation. Our initial work studied transfer of micronutrients such as copper and iron. We identified several steps in the transfer process, how they adapt to changing nutritional environment and how the transfer is regulated (for reviews see Andersen & McArdle, 2004; Gambling & McArdle, 2004). Recently, we have been examining how different nutrients interact during pregnancy. Iron and copper have long been known to influence each other. In the liver, copper deficiency results in an increase in iron levels while iron deficiency increases copper, at least in the maternal liver. In the fetal liver, in contrast, maternal iron deficiency induces a decrease, not an increase, in copper levels(Gambling et al., 2004). Associated with these changes are alterations in the expression of genes regulating iron status while mRNA levels of genes of copper metabolism do not change. This fits with their regulation taking place through altered cellular location rather than changes in transcription. The regulation of iron transfer across the placenta is complex. We demonstrated, using an iron deficiency model, a hierarchy of delivery. The fetal liver drives the regulation, with levels being maintained at the cost of the mother. Second in the hierarchy comes the maternal blood supply and maternal stores third. The data explain why, in humans, a mother will lose as much as 300 mg of iron from her stores with every pregnancy (Bothwell, 2000). Changing nutrient status, however, has wider consequences than just for that nutrient. We have used the BeWo cell line as a model for placental function for many years. More recently we started using the b30 clone, which has several advantages over the ATCC clone we used originally. This clone has been shown by several groups to demonstrate differentiation and transport properties similar to placenta (Moe et al., 1994) and has been used to examine polarisation and transport across the cell layer (Liu et al., 1997). We have used these cells as part of an integrated approach to understanding modulation of transfer of nutrients. Our initial experiments studied amino acid transporter systems, specifically those involved in System A, which transports amino acids such as alanine and glycine, Using the b30 cells, we were able to adopt an integrated approach, examining transcellular transfer, gene and protein expression in the same experiments. We grew the cells on filters and measured transcellular flux, then used immunohistochemistry, Northern and western blotting to determine protein and gene expression. We have used 14C-labelled Methyl amino isobutyric acid (Me-AIB) as a non-metabolisable substrate for System A. Changing amino acid levels in the apical medium results in an increase in amino acid transfer. This is associated with a change in localisation of SNAT2, one of the major System A proteins. Longer periods of deprivation result in increased synthesis of SNAT2, both at mRNA and protein levels (Jones et al., 2006). Importantly, deprivation of other nutrients will also stimulate these responses. We have shown that both iron and copper deficiency induce increased transfer of amino acids, together with the changes described in the previous paragraph. The implication, which is potentially very significant, is that increased amino acid transfer across the placenta is a generic stress response, rather than a particular change in response to alterations in one nutrient (Jones et al., 2006). Studying how nutrients interact in generating changes in placental function led us to develop a placental cDNA array, with which we could identify a wider number of genes altered by nutrient stress. We tested whether the same genes and gene pathways would be modulated by nutritional and endocrine effectors. In summary, we demonstrated that they were not. When we used amino acid restriction, we had genes both up- and down-regulated. Using cortisol as an endocrine inducer gave a very different response. In fact, only two genes were affected in common by the two stressors, and of these, one was regulated in the opposite manner. In summary, it is clear that transplacental transfer of nutrients is a well regulated and efficient process. This has, of course, been accepted for many years. Now, however, we also have to take into account the cross talk between different substrates, and have to elucidate the mechanisms underpinning the interactions. This promises a whole new and exciting arena for placental transport studies. Acknowledgements: This work was supported by SEERAD, the European Union and the International Copper AssociationReference 1 : Andersen, HS & McArdle, HJ (2004). How are genes measured? Examples from studies on iron metabolism in pregnancy. Proc. Nut. Soc. 63, 481-490.Reference 2 : Bothwell, TH (2000). Iron requirements in pregnancy and strategies to meet them. Am J Clin Nutr 72, 257S-264SReference 3 : Gambling, L, Dunford, S & McArdle, HJ (2004). Iron deficiency in the pregnant rat has differential effects on maternal and fetal copper levels. J Nutr Biochem 15, 366-372.Reference 4 : Gambling, L & McArdle, HJ (2004). Iron, copper and fetal development. Proc Nutr Soc 63, 553-562Reference 5 : Jones, HN, Ashworth, CJ, Page, KR & McArdle, HJ (2006b). Expression and adaptive regulation of amino acid transport system A in a placental cell line under amino acid restriction. Reprod. 131, 591-560.Reference 6 : Liu, F, Soares, MJ & Audus, KL (1997). Permeability properties of monolayers of the human trophoblast cell line BeWo. Am J Physiol 273, C1596-1604.Reference 7 : Moe, AJ, Furesz, TC & Smith, C (1994). Functional characterization of L-alanine transport in a placental choriocarcinoma cell line (BeWo). Placenta 15, 797-802.