The organic anion transporter (OAT) and organic anion transporter polypeptide (OATP) families mediate exchange of xenobiotics, hormones and drugs, including statins, antibiotics and anti-cancer drugs, whereby uptake of substrates is coupled to the efflux of counter-ions. To accumulate substrates in the cell by exchange, the counter-ion must have a higher concentration in the cell compared to the substrate. The gradients which drive OAT/OATP function are unknown but using Xenopus laevis efflux studies, we have demonstrated that OAT4 (SLC22A11) and OATP2B1 (SLCO2B1) transport glutamate and that in the placenta, glutamate is exchanged for organic anions. Previously OAT2 (SLC22A8) and OAT10 (SLC22A13) have been shown to transport glutamate and this raises questions about other members of these families. This study uses a screening approach to investigate the other members of the OAT/OATP families’ affinity for glutamate. Xenopus laevis oocytes were microinjected with water or with 20 μg human cRNA of either: OAT3, OAT5, OAT7, OATP2A1, OATP2B1 or OATP4A1 and incubated for 48 hours. Individual oocytes were placed into wells of a 96 well plate and incubated with the prototypical substrate, 3H-estrone-sulphate (11 μM), for 10 min with and without 2.5 mM glutamate or 2.5 mM estrone-sulphate (n=10 oocytes per condition). Uptake was stopped by washing in ice-cold ND91 buffer and 3H-estrone-sulphate uptake was determined via liquid scintillation counting following single oocyte homogenisation. Data are presented as mean (SEM), n=3 experiments. Data were analysed using a one-way ANOVA with a Dunnett’s post hoc test and significance was assumed when p<0.05. Estrone-sulphate inhibited 3H-estrone-sulphate uptake in OAT3, OAT7, OATP2A1, OATP2B1 and OATP4A1 injected oocytes (p<0.05) indicating that the transporters were functioning as expected. Glutamate inhibited 3H-estrone-sulphate uptake in OATP2B1 and OATP4A1 oocytes (p<0.001) but did not alter uptake in OAT3, OAT5, OAT7 or OATP2A1 injected oocytes. We have used a screening method to assess OAT/OATP glutamate affinity, which we have used to study six transporters simultaneously. In addition to OAT2, OAT4, OAT10 and OATP2B1, we propose that glutamate is transported by or inhibits OATP4A1, which will now be investigated further. OAT/OATP transporters have important biological roles throughout the body, particularly in the kidneys, liver and placenta. Understanding what drives the activity of these transporters may thus have implications for pharmacokinetic modelling of drug transport throughout the body.