System N (SNAT3 and SNAT5) amino acid transporters are key mediators of glutamine transport across the plasma membrane of mammalian cell types, including hepatocytes and astrocytes. We demonstrate that SNAT5 shows simultaneous bidirectional glutamine fluxes when overexpressed in Xenopus oocytes. Influx and efflux are both apparently Na+ dependent but, since they are not directly coupled, the carrier is capable of mediating net amino acid movement across the cell membrane. The apparent K-m values for glutamine influx and efflux are similar (similar to 1 mm) and the transporter behaviour is consistent with a kinetic model in which re-orientation of the carrier from outside- to inside-facing conformations (either empty or substrate loaded) is the limiting step in the transport cycle. In perfused rat liver, the observed relationship between influent (portal) glutamine concentration and net hepatic glutamine flux may be described by a simple kinetic model, assuming the balance between influx and efflux through System N determines net flux, where under physiological conditions efflux is generally saturated owing to high intracellular glutamine concentration. SNAT5 shows a more periportal mRNA distribution than SNAT3 in rat liver, indicating that SNAT5 may have particular importance for modulation of net hepatic glutamine flux.