Membrane currents produced by the expression of a rat GABA transporter (GAT-1) stably transfected into HEK293 cells were characterized with a whole-cell voltage clamp. Three modes of function were identified: ex-gated currents produced by extracellular GABA, in-gated currents produced by intracellular GABA, and uncoupled currents produced in the absence of GABA. The ex-gated current was not the reversal of the in-gated current; moreover, the stoichiometry between GABA and co-ions was not always fixed. Each mode of function required a different set of ions on the two sides of the membrane. We made rapid solution changes and observed an allosteric effect of Na+ that only occurred at the extracellular surface. Thus, the GAT-1 transporter does not behave like a recirculating carrier but may be described as a pore with ion gates at either end that are controlled in part by allosteric sites.