In the central nervous system (CNS), glutamate is the predominant excitatory neurotransmitter. Glutamate transporters remove the transmitter from the cleft and maintain its extracellular concentrations below neurotoxic levels (1–5). In addition, at some synapses glutamate transporters play an important role in limiting the duration of synaptic excitation (6–9). Glutamate uptake is an electrogenic process (10,11) in which the transmitter is cotransported with three sodium ions and a proton (3,12),followed by countertransport of a potassium ion (13–15). In addition to this coupled flux, glutamate transporters mediate a thermodynamically uncoupled chloride flux, activated by two of the molecules they transport: sodium and glutamate (16–18). This indicates the existence of a tight link between gating of the anion conductance and permeation of glutamate. It has been suggested that this capacity for enhancing chloride permeability could alter neuronal excitability (17).