Amino acids added to embryo culture medium are known to improve development and maintain viability of mouse embryos in vitro. Addition of specific individual amino acids, such as L-glutamine and L-proline, to culture medium significantly improves development of zygotes to blastocyst stage, while other amino acids have no effect. Preliminary results suggest that the improvement in development is not due to L-proline acting as an organic osmolyte but as a signalling-like growth factor. The positive bioactive action of L-proline on early embryo development relies on the expression of proline transporters for L-proline to be taken up into the embryo. Proline is present in mouse oviductal fluid in vivo (Guerin et al., 1995a) and is accumulated in mouse embryos when added to culture media in vitro. At least some of the L-proline accumulation in zygotes can be attributed to expression of the transporter SIT1, which actively transports the L-proline into the embryo after fertilisation until the 2-cell stage (Anas et al., 2008). There is also at least one other unknown transporter that requires sodium and is betaine resistant involved in this process (Anas et a., 2008). Possible candidates for proline transport in oocytes was investigated by measuring uptake of radiolabelled L-proline in the presence of competitive amino acid. Uptake of L-proline was observed in oocytes and this was inhibited by the presence of excess L-pipecolic Acid and histidine, suggesting PROT (slc6a7) is the transporter responsible. Immunostaining of PROT showed localisation to the plasma membrane in oocytes. The role of PROT expression in oocytes is unknown but the expression of different amino acid transporters in the oocyte and at each embryonic stage may reflect the changes in amino acid requirements during early development. These findings may impact on the embryo culture routinely used in Assisted Reproductive Technologies.