System β is a Na⁺-dependent amino acid transporter specific for taurine and other β amino acids. It is encoded by the TauT gene which has been cloned from human placenta¹ and identified at protein level in the syncytiotrophoblast². System β is functionally active in the placenta³ and is responsible for transport of taurine from mother to fetus. Taurine is regarded as an essential amino acid during fetal growth because the human fetus has poor synthesis capabilities⁴. As pregnancy advances and growth of the fetus accelerates, demand for nutrient supply increases. Here we test the hypothesis that placental System β gene expression and activity increases over gestation in relation to the rising fetal requirement. First trimester (6-13 weeks) and term (38-40 weeks) placentas were collected within 30min of delivery. In order to measure TauT gene expression, placental tissue was homogenised and total RNA extracted to generate the cDNA template for QPCR. TauT mRNA was measured using the intercalating dye SYBR Green 1, quantified from a Human Reference Total RNA standard curve and normalised to a term placental calibrator sample. System β activity was measured by incubating fresh villous tissue (2-3mm³)at 37°C in Tyrodes solution (+/-Na⁺;choline replaced Na⁺) containing 30nM ³H-taurine. After specified time periods, uptake was terminated by washing in ice cold Tyrodes. Fragments were lysed in distilled water and the lysate counted for radioactivity. Fragments were then dissolved in NaOH and assayed for protein content. To distinguish transport of taurine into the tissue by system β from non-specific accumulation, the difference between uptake in the presence and absence of Na⁺ was calculated. This represents the Na⁺-dependent component of taurine uptake by system β and is expressed in fmol per mg protein. These results show that mRNA expression and Na⁺-dependent activity of the System β amino acid transporter is present in first trimester placenta and is not different to that at term.