The mechanisms mediating amino acid transport across the basal membrane of the placental syncytiotrophoblast and into the fetal circulation are not well understood. This study investigates the hypothesis that exchange transporters may mediate the process of amino acid transport in the placenta. Specifically, the mechanisms by which serine and glycine transfer occurs across the isolated perfused human placental cotyledon are investigated. Placentas (n = 5) were collected within 30 min of delivery and perfusion of an intact cotyledon with a modified Earl’s Bicarbonate Buffer established as described previously (Brownbill et al. 1995). Perfusion was considered to be successful where there was > 95% recovery of fetal perfusate. The maternal arterial circulation was perfused with 100 µmol/l L-serine, glycine, 0.6 µmol/l ¹⁴C-L-serine, 20 µmol/l ³H-glycine and 1.8 mM creatinine. The role of exchangers in the transport of serine and glycine was assessed by giving of glutamate (20 µmol) and alanine (10 and 20 µmol) boluses to the fetal arterial circulation. The rationale was that alanine is a substrate for amino acid exchangers and so should stimulate serine/glycine transfer, while glutamate is not an exchanger substrate and should not stimulate transfer. Appearance of ¹⁴C-L-serine and ³H-glycine were measured in maternal and fetal venous samples by dual label liquid scintillation counting. Data (mean ± SEM) were expressed as area under the curve (AUC) and analysed by one-way ANOVA followed by Bonferroni post hoc test. Under our basal conditions, the proportion of maternal substrate appearing in the fetal venous circulation for serine (8.6 ± 1.0%) and glycine (8.3 ± 0.41%) was similar to that of creatinine (8.2 ± 1.1%), a marker of transfer by paracellular routes. There was no significant increase in transfer of serine or glycine following a fetal arterial bolus of 20 µmol glutamate (serine 0.9 ± 1.1 nmol and glycine 2.5 ± 3.1 nmol). These values were then used as baseline for analysis of the effects of alanine boluses. Following a 10 µmol alanine bolus there was an increase in serine release (AUC 29.4 ± 5.1 nmol, p < 0.05). Serine release following a 20 µmol alanine bolus was greater than baseline and that following the 10 µmol alanine bolus (AUC 61.0 ± 5.8 nmol, p < 0.05). Glycine release was not significantly altered following 10 or 20 µmol alanine boluses (AUC 8.4 ± 2.8 and 5.5 ± 2.5 nmol, respectively). The increased transport of serine into the fetal circulation following increasing doses of alanine demonstrates for the first time that amino acid exchangers mediate serine transfer from the placenta into the fetal circulation in the intact human cotyledon. The same mechanism does not appear to operate for glycine transfer.