High affinity glutamate receptors may be activated by the baseline extracellular glutamate concentration, [Glu]_o. The most abundant glutamate transporter, GLT-1, could lower [Glu]_o to ~2nM, based on its ionic stoichiometry and the concentrations of its driving ions, Na⁺, K⁺ and H⁺, but measurements using microdialysis give a value of ~1µM (Cavelier et al. 2005). If other glutamate transporters had a different stoichiometry, the minimum glutamate concentration could be higher, for example if those transporters were driven by the co-transport of 2 Na⁺ (rather than of 3 Na⁺ as for GLT-1). Here we investigated the ionic stoichiometry of the glutamate transporter GLAST, which is highly expressed in the retina and cerebellum, and is present throughout the brain early in development when expression of GLT-1 is low. Müller cells from salamander retina were dissociated and whole-cell patch-clamped to monitor glutamate transport as a membrane current (Brew & Attwell, 1987). These cells express homologues of the mammalian transporters GLAST, GLT-1 and EAAT5 (Eliasof et al. 1997), but the low sensitivity of their transport current to dihydrokainate and the absence of a large transporter anion conductance imply that GLAST is the only transporter that generates a significant current. The internal and external solutions lacked Cl^– ions to abolish currents generated by the transporter’s anion conductance. With glutamate, Na⁺, K⁺ and H⁺ present on both sides of the membrane the direction of transport at different voltages was assessed by blocking transporter action with TBOA (cf. Levy et al. 1998). The reversal potential for the TBOA-blocked current is determined by the transporter stoichiometry. We measured the transporter reversal potential using a standard pair of intra- and extracellular solutions, and then with the external [Na⁺] reduced by 1/3, the external [glutamate] increased 5-fold, the external [H⁺] increased 2.5-fold, or the external [K⁺] decreased 2.5-fold. For all these manipulations the measured change of reversal potential was within a few mV of (not significantly different from) that predicted for a stoichiometry in which each glutamate anion is co-transported with 3 Na⁺ and 1 H⁺ while 1 K⁺ is counter-transported, as previously found for GLT-1 and EAAC1 (Zerangue & Kavanaugh, 1996; Levy et al. 1998), and was significantly different to the prediction for the case of 2 Na⁺ being transported. The demonstration that the stoichiometry of GLAST is the same as that for GLT-1 suggests that the minimum extracellular glutamate concentration should be similar during development and in the adult brain. A less powerful accumulation of glutamate by GLAST than by GLT-1 cannot be used to explain the high glutamate concentration measured by microdialysis.